The functions in this section can serve as terms in an expression.
       They fall into two major categories: list operators and named unary
       operators.  These differ in their precedence relationship with a
       following comma.  (See the precedence table in perlop.)  List operators
       take more than one argument, while unary operators can never take more
       than one argument.  Thus, a comma terminates the argument of a unary
       operator, but merely separates the arguments of a list operator.  A
       unary operator generally provides scalar context to its argument, while
       a list operator may provide either scalar or list contexts for its
       arguments.  If it does both, scalar arguments come first and list
       argument follow, and there can only ever be one such list argument.
       For instance, splice() has three scalar arguments followed by a list,
       whereas gethostbyname() has four scalar arguments.

       In the syntax descriptions that follow, list operators that expect a
       list (and provide list context for elements of the list) are shown with
       LIST as an argument.  Such a list may consist of any combination of
       scalar arguments or list values; the list values will be included in
       the list as if each individual element were interpolated at that point
       in the list, forming a longer single-dimensional list value.  Commas
       should separate literal elements of the LIST.

       Any function in the list below may be used either with or without
       parentheses around its arguments.  (The syntax descriptions omit the
       parentheses.)  If you use parentheses, the simple but occasionally
       surprising rule is this: It looks like a function, therefore it is a
       function, and precedence doesn't matter.  Otherwise it's a list
       operator or unary operator, and precedence does matter.  Whitespace
       between the function and left parenthesis doesn't count, so sometimes
       you need to be careful:

           print 1+2+4;      # Prints 7.
           print(1+2) + 4;   # Prints 3.
           print (1+2)+4;    # Also prints 3!
           print +(1+2)+4;   # Prints 7.
           print ((1+2)+4);  # Prints 7.

       If you run Perl with the -w switch it can warn you about this.  For
       example, the third line above produces:

           print (...) interpreted as function at - line 1.
           Useless use of integer addition in void context at - line 1.

       A few functions take no arguments at all, and therefore work as neither
       unary nor list operators.  These include such functions as "time" and
       "endpwent".  For example, "time+86_400" always means "time() + 86_400".

       For functions that can be used in either a scalar or list context,
       nonabortive failure is generally indicated in scalar context by
       returning the undefined value, and in list context by returning the
       empty list.

       first glance appear to be a list in scalar context.  You can't get a
       list like "(1,2,3)" into being in scalar context, because the compiler
       knows the context at compile time.  It would generate the scalar comma
       operator there, not the list construction version of the comma.  That
       means it was never a list to start with.

       In general, functions in Perl that serve as wrappers for system calls
       ("syscalls") of the same name (like chown(2), fork(2), closedir(2),
       etc.) return true when they succeed and "undef" otherwise, as is
       usually mentioned in the descriptions below.  This is different from
       the C interfaces, which return "-1" on failure.  Exceptions to this
       rule include "wait", "waitpid", and "syscall".  System calls also set
       the special $!  variable on failure.  Other functions do not, except

       Extension modules can also hook into the Perl parser to define new
       kinds of keyword-headed expression.  These may look like functions, but
       may also look completely different.  The syntax following the keyword
       is defined entirely by the extension.  If you are an implementor, see
       "PL_keyword_plugin" in perlapi for the mechanism.  If you are using
       such a module, see the module's documentation for details of the syntax
       that it defines.

   Perl Functions by Category
       Here are Perl's functions (including things that look like functions,
       like some keywords and named operators) arranged by category.  Some
       functions appear in more than one place.

       Functions for SCALARs or strings
           "chomp", "chop", "chr", "crypt", "hex", "index", "lc", "lcfirst",
           "length", "oct", "ord", "pack", "q//", "qq//", "reverse", "rindex",
           "sprintf", "substr", "tr///", "uc", "ucfirst", "y///"

       Regular expressions and pattern matching
           "m//", "pos", "quotemeta", "s///", "split", "study", "qr//"

       Numeric functions
           "abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand",
           "sin", "sqrt", "srand"

       Functions for real @ARRAYs
           "each", "keys", "pop", "push", "shift", "splice", "unshift",

       Functions for list data
           "grep", "join", "map", "qw//", "reverse", "sort", "unpack"

       Functions for real %HASHes
           "delete", "each", "exists", "keys", "values"

       Input and output functions
           "binmode", "close", "closedir", "dbmclose", "dbmopen", "die",
           "eof", "fileno", "flock", "format", "getc", "print", "printf",
           "read", "readdir", "rewinddir", "say", "seek", "seekdir", "select",

       Keywords related to the control flow of your Perl program
           "caller", "continue", "die", "do", "dump", "eval", "exit", "goto",
           "last", "next", "redo", "return", "sub", "wantarray"

       Keywords related to the switch feature
           "break", "continue", "default, ""given", "when"

           These are available only if you enable the "switch" feature.  See
           feature and "Switch statements" in perlsyn.  Alternately, include a
           "use v5.10" or later to the current scope.

       Keywords related to scoping
           "caller", "import", "local", "my", "our", "package", "state", "use"

           "state" is available only if the "state" feature is enabled. See
           feature.  Alternately, include a "use v5.10" or later to the
           current scope.

       Miscellaneous functions
           "defined", "dump", "eval", "formline", "local", "my", "our",
           "reset", "scalar", "state", "undef", "wantarray"

       Functions for processes and process groups
           "alarm", "exec", "fork", "getpgrp", "getppid", "getpriority",
           "kill", "pipe", "qx//", "setpgrp", "setpriority", "sleep",
           "system", "times", "wait", "waitpid"

       Keywords related to Perl modules
           "do", "import", "no", "package", "require", "use"

       Keywords related to classes and object-orientation
           "bless", "dbmclose", "dbmopen", "package", "ref", "tie", "tied",
           "untie", "use"

       Low-level socket functions
           "accept", "bind", "connect", "getpeername", "getsockname",
           "getsockopt", "listen", "recv", "send", "setsockopt", "shutdown",
           "socket", "socketpair"

       System V interprocess communication functions
           "msgctl", "msgget", "msgrcv", "msgsnd", "semctl", "semget",
           "semop", "shmctl", "shmget", "shmread", "shmwrite"

       Fetching user and group info
           "endgrent", "endhostent", "endnetent", "endpwent", "getgrent",
           "getgrgid", "getgrnam", "getlogin", "getpwent", "getpwnam",
           "getpwuid", "setgrent", "setpwent"

       Fetching network info
           "endprotoent", "endservent", "gethostbyaddr", "gethostbyname",
           "gethostent", "getnetbyaddr", "getnetbyname", "getnetent",
           "getprotobyname", "getprotobynumber", "getprotoent",
           "getservbyname", "getservbyport", "getservent", "sethostent",
           "setnetent", "setprotoent", "setservent"
           which can be used in expressions.

       Functions obsoleted in perl5
           "dbmclose", "dbmopen"

       Perl was born in Unix and can therefore access all common Unix system
       calls.  In non-Unix environments, the functionality of some Unix system
       calls may not be available or details of the available functionality
       may differ slightly.  The Perl functions affected by this are:

       "-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose",
       "dbmopen", "dump", "endgrent", "endhostent", "endnetent",
       "endprotoent", "endpwent", "endservent", "exec", "fcntl", "flock",
       "fork", "getgrent", "getgrgid", "gethostbyname", "gethostent",
       "getlogin", "getnetbyaddr", "getnetbyname", "getnetent", "getppid",
       "getpgrp", "getpriority", "getprotobynumber", "getprotoent",
       "getpwent", "getpwnam", "getpwuid", "getservbyport", "getservent",
       "getsockopt", "glob", "ioctl", "kill", "link", "lstat", "msgctl",
       "msgget", "msgrcv", "msgsnd", "open", "pipe", "readlink", "rename",
       "select", "semctl", "semget", "semop", "setgrent", "sethostent",
       "setnetent", "setpgrp", "setpriority", "setprotoent", "setpwent",
       "setservent", "setsockopt", "shmctl", "shmget", "shmread", "shmwrite",
       "socket", "socketpair", "stat", "symlink", "syscall", "sysopen",
       "system", "times", "truncate", "umask", "unlink", "utime", "wait",

       For more information about the portability of these functions, see
       perlport and other available platform-specific documentation.

   Alphabetical Listing of Perl Functions
       -X EXPR
       -X  A file test, where X is one of the letters listed below.  This
           unary operator takes one argument, either a filename, a filehandle,
           or a dirhandle, and tests the associated file to see if something
           is true about it.  If the argument is omitted, tests $_, except for
           "-t", which tests STDIN.  Unless otherwise documented, it returns 1
           for true and '' for false, or the undefined value if the file
           doesn't exist.  Despite the funny names, precedence is the same as
           any other named unary operator.  The operator may be any of:

               -r  File is readable by effective uid/gid.
               -w  File is writable by effective uid/gid.
               -x  File is executable by effective uid/gid.
               -o  File is owned by effective uid.

               -R  File is readable by real uid/gid.
               -W  File is writable by real uid/gid.
               -X  File is executable by real uid/gid.
               -O  File is owned by real uid.

               -e  File exists.
               -u  File has setuid bit set.
               -g  File has setgid bit set.
               -k  File has sticky bit set.

               -T  File is an ASCII text file (heuristic guess).
               -B  File is a "binary" file (opposite of -T).

               -M  Script start time minus file modification time, in days.
               -A  Same for access time.
               -C  Same for inode change time (Unix, may differ for other platforms)


               while (<>) {
                   next unless -f $_;  # ignore specials

           Note that "-s/a/b/" does not do a negated substitution.  Saying
           "-exp($foo)" still works as expected, however: only single letters
           following a minus are interpreted as file tests.

           These operators are exempt from the "looks like a function rule"
           described above. That is, an opening parenthesis after the operator
           does not affect how much of the following code constitutes the
           argument. Put the opening parentheses before the operator to
           separate it from code that follows (this applies only to operators
           with higher precedence than unary operators, of course):

               -s($file) + 1024   # probably wrong; same as -s($file + 1024)
               (-s $file) + 1024  # correct

           The interpretation of the file permission operators "-r", "-R",
           "-w", "-W", "-x", and "-X" is by default based solely on the mode
           of the file and the uids and gids of the user.  There may be other
           reasons you can't actually read, write, or execute the file: for
           example network filesystem access controls, ACLs (access control
           lists), read-only filesystems, and unrecognized executable formats.
           Note that the use of these six specific operators to verify if some
           operation is possible is usually a mistake, because it may be open
           to race conditions.

           Also note that, for the superuser on the local filesystems, the
           "-r", "-R", "-w", and "-W" tests always return 1, and "-x" and "-X"
           return 1 if any execute bit is set in the mode.  Scripts run by the
           superuser may thus need to do a stat() to determine the actual mode
           of the file, or temporarily set their effective uid to something

           If you are using ACLs, there is a pragma called "filetest" that may
           produce more accurate results than the bare stat() mode bits.  When
           under "use filetest 'access'" the above-mentioned filetests test
           whether the permission can(not) be granted using the access(2)
           characters (>30%) are found, it's a "-B" file; otherwise it's a
           "-T" file.  Also, any file containing a zero byte in the first
           block is considered a binary file.  If "-T" or "-B" is used on a
           filehandle, the current IO buffer is examined rather than the first
           block.  Both "-T" and "-B" return true on an empty file, or a file
           at EOF when testing a filehandle.  Because you have to read a file
           to do the "-T" test, on most occasions you want to use a "-f"
           against the file first, as in "next unless -f $file && -T $file".

           If any of the file tests (or either the "stat" or "lstat" operator)
           is given the special filehandle consisting of a solitary underline,
           then the stat structure of the previous file test (or stat
           operator) is used, saving a system call.  (This doesn't work with
           "-t", and you need to remember that lstat() and "-l" leave values
           in the stat structure for the symbolic link, not the real file.)
           (Also, if the stat buffer was filled by an "lstat" call, "-T" and
           "-B" will reset it with the results of "stat _").  Example:

               print "Can do.\n" if -r $a || -w _ || -x _;

               print "Readable\n" if -r _;
               print "Writable\n" if -w _;
               print "Executable\n" if -x _;
               print "Setuid\n" if -u _;
               print "Setgid\n" if -g _;
               print "Sticky\n" if -k _;
               print "Text\n" if -T _;
               print "Binary\n" if -B _;

           As of Perl 5.9.1, as a form of purely syntactic sugar, you can
           stack file test operators, in a way that "-f -w -x $file" is
           equivalent to "-x $file && -w _ && -f _". (This is only fancy
           fancy: if you use the return value of "-f $file" as an argument to
           another filetest operator, no special magic will happen.)

       abs VALUE
       abs Returns the absolute value of its argument.  If VALUE is omitted,
           uses $_.

           Accepts an incoming socket connect, just as accept(2) does.
           Returns the packed address if it succeeded, false otherwise.  See
           the example in "Sockets: Client/Server Communication" in perlipc.

           On systems that support a close-on-exec flag on files, the flag
           will be set for the newly opened file descriptor, as determined by
           the value of $^F.  See "$^F" in perlvar.

       alarm SECONDS
           Arranges to have a SIGALRM delivered to this process after the
           specified number of wallclock seconds has elapsed.  If SECONDS is
           not specified, the value stored in $_ is used. (On some machines,
           distribution) provides ualarm().  You may also use Perl's four-
           argument version of select() leaving the first three arguments
           undefined, or you might be able to use the "syscall" interface to
           access setitimer(2) if your system supports it. See perlfaq8 for

           It is usually a mistake to intermix "alarm" and "sleep" calls,
           because "sleep" may be internally implemented on your system with

           If you want to use "alarm" to time out a system call you need to
           use an "eval"/"die" pair.  You can't rely on the alarm causing the
           system call to fail with $! set to "EINTR" because Perl sets up
           signal handlers to restart system calls on some systems.  Using
           "eval"/"die" always works, modulo the caveats given in "Signals" in

               eval {
                   local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
                   alarm $timeout;
                   $nread = sysread SOCKET, $buffer, $size;
                   alarm 0;
               if ($@) {
                   die unless $@ eq "alarm\n";   # propagate unexpected errors
                   # timed out
               else {
                   # didn't

           For more information see perlipc.

       atan2 Y,X
           Returns the arctangent of Y/X in the range -PI to PI.

           For the tangent operation, you may use the "Math::Trig::tan"
           function, or use the familiar relation:

               sub tan { sin($_[0]) / cos($_[0])  }

           The return value for "atan2(0,0)" is implementation-defined;
           consult your atan2(3) manpage for more information.

       bind SOCKET,NAME
           Binds a network address to a socket, just as bind(2) does.  Returns
           true if it succeeded, false otherwise.  NAME should be a packed
           address of the appropriate type for the socket.  See the examples
           in "Sockets: Client/Server Communication" in perlipc.

       binmode FILEHANDLE, LAYER
       binmode FILEHANDLE
           Arranges for FILEHANDLE to be read or written in "binary" or "text"
           mode on systems where the run-time libraries distinguish between
           data, like images, for example.

           If LAYER is present it is a single string, but may contain multiple
           directives. The directives alter the behaviour of the filehandle.
           When LAYER is present, using binmode on a text file makes sense.

           If LAYER is omitted or specified as ":raw" the filehandle is made
           suitable for passing binary data. This includes turning off
           possible CRLF translation and marking it as bytes (as opposed to
           Unicode characters).  Note that, despite what may be implied in
           "Programming Perl" (the Camel, 3rd edition) or elsewhere, ":raw" is
           not simply the inverse of ":crlf".  Other layers that would affect
           the binary nature of the stream are also disabled. See PerlIO,
           perlrun, and the discussion about the PERLIO environment variable.

           The ":bytes", ":crlf", ":utf8", and any other directives of the
           form ":...", are called I/O layers.  The "open" pragma can be used
           to establish default I/O layers.  See open.

           The LAYER parameter of the binmode() function is described as
           "DISCIPLINE" in "Programming Perl, 3rd Edition".  However, since
           the publishing of this book, by many known as "Camel III", the
           consensus of the naming of this functionality has moved from
           "discipline" to "layer".  All documentation of this version of Perl
           therefore refers to "layers" rather than to "disciplines".  Now
           back to the regularly scheduled documentation...

           To mark FILEHANDLE as UTF-8, use ":utf8" or ":encoding(UTF-8)".
           ":utf8" just marks the data as UTF-8 without further checking,
           while ":encoding(UTF-8)" checks the data for actually being valid
           UTF-8. More details can be found in PerlIO::encoding.

           In general, binmode() should be called after open() but before any
           I/O is done on the filehandle.  Calling binmode() normally flushes
           any pending buffered output data (and perhaps pending input data)
           on the handle.  An exception to this is the ":encoding" layer that
           changes the default character encoding of the handle; see "open".
           The ":encoding" layer sometimes needs to be called in mid-stream,
           and it doesn't flush the stream.  The ":encoding" also implicitly
           pushes on top of itself the ":utf8" layer because internally Perl
           operates on UTF8-encoded Unicode characters.

           The operating system, device drivers, C libraries, and Perl run-
           time system all conspire to let the programmer treat a single
           character ("\n") as the line terminator, irrespective of external
           representation.  On many operating systems, the native text file
           representation matches the internal representation, but on some
           platforms the external representation of "\n" is made up of more
           than one character.

           All variants of Unix, Mac OS (old and new), and Stream_LF files on
           VMS use a single character to end each line in the external
           representation of text (even though that single character is
           CARRIAGE RETURN on old, pre-Darwin flavors of Mac OS, and is LINE
           your binary data contain "\cZ", the I/O subsystem will regard it as
           the end of the file, unless you use binmode().

           binmode() is important not only for readline() and print()
           operations, but also when using read(), seek(), sysread(),
           syswrite() and tell() (see perlport for more details).  See the $/
           and "$\" variables in perlvar for how to manually set your input
           and output line-termination sequences.

       bless REF,CLASSNAME
       bless REF
           This function tells the thingy referenced by REF that it is now an
           object in the CLASSNAME package.  If CLASSNAME is omitted, the
           current package is used.  Because a "bless" is often the last thing
           in a constructor, it returns the reference for convenience.  Always
           use the two-argument version if a derived class might inherit the
           function doing the blessing.  See perltoot and perlobj for more
           about the blessing (and blessings) of objects.

           Consider always blessing objects in CLASSNAMEs that are mixed case.
           Namespaces with all lowercase names are considered reserved for
           Perl pragmata.  Builtin types have all uppercase names. To prevent
           confusion, you may wish to avoid such package names as well.  Make
           sure that CLASSNAME is a true value.

           See "Perl Modules" in perlmod.

           Break out of a "given()" block.

           This keyword is enabled by the "switch" feature: see feature for
           more information.  Alternately, include a "use v5.10" or later to
           the current scope.

       caller EXPR
           Returns the context of the current subroutine call.  In scalar
           context, returns the caller's package name if there is a caller
           (that is, if we're in a subroutine or "eval" or "require") and the
           undefined value otherwise.  In list context, returns

               # 0         1          2
               ($package, $filename, $line) = caller;

           With EXPR, it returns some extra information that the debugger uses
           to print a stack trace.  The value of EXPR indicates how many call
           frames to go back before the current one.

               #  0         1          2      3            4
               ($package, $filename, $line, $subroutine, $hasargs,

               #  5          6          7            8       9         10
               $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
                = caller($i);

           pragmatic hints that the caller was compiled with.  The $hints and
           $bitmask values are subject to change between versions of Perl, and
           are not meant for external use.

           $hinthash is a reference to a hash containing the value of "%^H"
           when the caller was compiled, or "undef" if "%^H" was empty. Do not
           modify the values of this hash, as they are the actual values
           stored in the optree.

           Furthermore, when called from within the DB package, caller returns
           more detailed information: it sets the list variable @DB::args to
           be the arguments with which the subroutine was invoked.

           Be aware that the optimizer might have optimized call frames away
           before "caller" had a chance to get the information.  That means
           that caller(N) might not return information about the call frame
           you expect it to, for "N > 1".  In particular, @DB::args might have
           information from the previous time "caller" was called.

           Be aware that setting @DB::args is best effort, intended for
           debugging or generating backtraces, and should not be relied upon.
           In particular, as @_ contains aliases to the caller's arguments,
           Perl does not take a copy of @_, so @DB::args will contain
           modifications the subroutine makes to @_ or its contents, not the
           original values at call time. @DB::args, like @_, does not hold
           explicit references to its elements, so under certain cases its
           elements may have become freed and reallocated for other variables
           or temporary values. Finally, a side effect of the current
           implementation is that the effects of "shift @_" can normally be
           undone (but not "pop @_" or other splicing, and not if a reference
           to @_ has been taken, and subject to the caveat about reallocated
           elements), so @DB::args is actually a hybrid of the current state
           and initial state of @_. Buyer beware.

       chdir EXPR
       chdir FILEHANDLE
       chdir DIRHANDLE
           Changes the working directory to EXPR, if possible. If EXPR is
           omitted, changes to the directory specified by $ENV{HOME}, if set;
           if not, changes to the directory specified by $ENV{LOGDIR}. (Under
           VMS, the variable $ENV{SYS$LOGIN} is also checked, and used if it
           is set.) If neither is set, "chdir" does nothing. It returns true
           on success, false otherwise. See the example under "die".

           On systems that support fchdir(2), you may pass a filehandle or
           directory handle as the argument.  On systems that don't support
           fchdir(2), passing handles raises an exception.

       chmod LIST
           Changes the permissions of a list of files.  The first element of
           the list must be the numeric mode, which should probably be an
           octal number, and which definitely should not be a string of octal
           digits: 0644 is okay, but "0644" is not.  Returns the number of
           filehandles raises an exception.  Filehandles must be passed as
           globs or glob references to be recognized; barewords are considered

               open(my $fh, "<", "foo");
               my $perm = (stat $fh)[2] & 07777;
               chmod($perm | 0600, $fh);

           You can also import the symbolic "S_I*" constants from the "Fcntl"

               use Fcntl qw( :mode );
               chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
               # Identical to the chmod 0755 of the example above.

       chomp VARIABLE
       chomp( LIST )
           This safer version of "chop" removes any trailing string that
           corresponds to the current value of $/ (also known as
           $INPUT_RECORD_SEPARATOR in the "English" module).  It returns the
           total number of characters removed from all its arguments.  It's
           often used to remove the newline from the end of an input record
           when you're worried that the final record may be missing its
           newline.  When in paragraph mode ("$/ = """), it removes all
           trailing newlines from the string.  When in slurp mode ("$/ =
           undef") or fixed-length record mode ($/ is a reference to an
           integer or the like; see perlvar) chomp() won't remove anything.
           If VARIABLE is omitted, it chomps $_.  Example:

               while (<>) {
                   chomp;  # avoid \n on last field
                   @array = split(/:/);
                   # ...

           If VARIABLE is a hash, it chomps the hash's values, but not its

           You can actually chomp anything that's an lvalue, including an

               chomp($cwd = `pwd`);
               chomp($answer = <STDIN>);

           If you chomp a list, each element is chomped, and the total number
           of characters removed is returned.

           Note that parentheses are necessary when you're chomping anything
           that is not a simple variable.  This is because "chomp $cwd =
           `pwd`;" is interpreted as "(chomp $cwd) = `pwd`;", rather than as
           "chomp( $cwd = `pwd` )" which you might expect.  Similarly, "chomp
           $a, $b" is interpreted as "chomp($a), $b" rather than as "chomp($a,

           If you chop a list, each element is chopped.  Only the value of the
           last "chop" is returned.

           Note that "chop" returns the last character.  To return all but the
           last character, use "substr($string, 0, -1)".

           See also "chomp".

       chown LIST
           Changes the owner (and group) of a list of files.  The first two
           elements of the list must be the numeric uid and gid, in that
           order.  A value of -1 in either position is interpreted by most
           systems to leave that value unchanged.  Returns the number of files
           successfully changed.

               $cnt = chown $uid, $gid, 'foo', 'bar';
               chown $uid, $gid, @filenames;

           On systems that support fchown(2), you may pass filehandles among
           the files.  On systems that don't support fchown(2), passing
           filehandles raises an exception.  Filehandles must be passed as
           globs or glob references to be recognized; barewords are considered

           Here's an example that looks up nonnumeric uids in the passwd file:

               print "User: ";
               chomp($user = <STDIN>);
               print "Files: ";
               chomp($pattern = <STDIN>);

               ($login,$pass,$uid,$gid) = getpwnam($user)
                   or die "$user not in passwd file";

               @ary = glob($pattern);  # expand filenames
               chown $uid, $gid, @ary;

           On most systems, you are not allowed to change the ownership of the
           file unless you're the superuser, although you should be able to
           change the group to any of your secondary groups.  On insecure
           systems, these restrictions may be relaxed, but this is not a
           portable assumption.  On POSIX systems, you can detect this
           condition this way:

               use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
               $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);

       chr NUMBER
       chr Returns the character represented by that NUMBER in the character
           set.  For example, "chr(65)" is "A" in either ASCII or Unicode, and
           chr(0x263a) is a Unicode smiley face.

           Negative values give the Unicode replacement character

       chroot FILENAME
           This function works like the system call by the same name: it makes
           the named directory the new root directory for all further
           pathnames that begin with a "/" by your process and all its
           children.  (It doesn't change your current working directory, which
           is unaffected.)  For security reasons, this call is restricted to
           the superuser.  If FILENAME is omitted, does a "chroot" to $_.

       close FILEHANDLE
           Closes the file or pipe associated with the filehandle, flushes the
           IO buffers, and closes the system file descriptor.  Returns true if
           those operations succeed and if no error was reported by any PerlIO
           layer.  Closes the currently selected filehandle if the argument is

           You don't have to close FILEHANDLE if you are immediately going to
           do another "open" on it, because "open" closes it for you.  (See
           "open".)  However, an explicit "close" on an input file resets the
           line counter ($.), while the implicit close done by "open" does

           If the filehandle came from a piped open, "close" returns false if
           one of the other syscalls involved fails or if its program exits
           with non-zero status.  If the only problem was that the program
           exited non-zero, $!  will be set to 0.  Closing a pipe also waits
           for the process executing on the pipe to exit--in case you wish to
           look at the output of the pipe afterwards--and implicitly puts the
           exit status value of that command into $? and

           If there are multiple threads running, "close" on a filehandle from
           a piped open returns true without waiting for the child process to
           terminate, if the filehandle is still open in another thread.

           Closing the read end of a pipe before the process writing to it at
           the other end is done writing results in the writer receiving a
           SIGPIPE.  If the other end can't handle that, be sure to read all
           the data before closing the pipe.


               open(OUTPUT, '|sort >foo')  # pipe to sort
                   or die "Can't start sort: $!";
               #...                        # print stuff to output
               close OUTPUT                # wait for sort to finish
                   or warn $! ? "Error closing sort pipe: $!"
                              : "Exit status $? from sort";
               open(INPUT, 'foo')          # get sort's results
                   or die "Can't open 'foo' for input: $!";

           FILEHANDLE may be an expression whose value can be used as an
           indirect filehandle, usually the real filehandle name or an

       continue BLOCK
           "continue" is actually a flow control statement rather than a
           function.  If there is a "continue" BLOCK attached to a BLOCK
           (typically in a "while" or "foreach"), it is always executed just
           before the conditional is about to be evaluated again, just like
           the third part of a "for" loop in C.  Thus it can be used to
           increment a loop variable, even when the loop has been continued
           via the "next" statement (which is similar to the C "continue"

           "last", "next", or "redo" may appear within a "continue" block;
           "last" and "redo" behave as if they had been executed within the
           main block.  So will "next", but since it will execute a "continue"
           block, it may be more entertaining.

               while (EXPR) {
                   ### redo always comes here
               } continue {
                   ### next always comes here
                   # then back the top to re-check EXPR
               ### last always comes here

           Omitting the "continue" section is equivalent to using an empty
           one, logically enough, so "next" goes directly back to check the
           condition at the top of the loop.

           If the "switch" feature is enabled, "continue" is also a function
           that falls through the current "when" or "default" block instead of
           iterating a dynamically enclosing "foreach" or exiting a lexically
           enclosing "given".  See feature and "Switch statements" in perlsyn
           for more information.

       cos EXPR
       cos Returns the cosine of EXPR (expressed in radians).  If EXPR is
           omitted, takes the cosine of $_.

           For the inverse cosine operation, you may use the
           "Math::Trig::acos()" function, or use this relation:

               sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }

       crypt PLAINTEXT,SALT
           Creates a digest string exactly like the crypt(3) function in the C
           library (assuming that you actually have a version there that has
           not been extirpated as a potential munition).

           crypt() is a one-way hash function.  The PLAINTEXT and SALT are
           turned into a short string, called a digest, which is returned.
           The same PLAINTEXT and SALT will always return the same string, but
           there is no (known) way to get the original PLAINTEXT from the

           When verifying an existing digest string you should use the digest
           as the salt (like "crypt($plain, $digest) eq $digest").  The SALT
           used to create the digest is visible as part of the digest.  This
           ensures crypt() will hash the new string with the same salt as the
           digest.  This allows your code to work with the standard crypt and
           with more exotic implementations.  In other words, assume nothing
           about the returned string itself nor about how many bytes of SALT
           may matter.

           Traditionally the result is a string of 13 bytes: two first bytes
           of the salt, followed by 11 bytes from the set "[./0-9A-Za-z]", and
           only the first eight bytes of PLAINTEXT mattered. But alternative
           hashing schemes (like MD5), higher level security schemes (like
           C2), and implementations on non-Unix platforms may produce
           different strings.

           When choosing a new salt create a random two character string whose
           characters come from the set "[./0-9A-Za-z]" (like "join '', ('.',
           '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]").  This set of
           characters is just a recommendation; the characters allowed in the
           salt depend solely on your system's crypt library, and Perl can't
           restrict what salts "crypt()" accepts.

           Here's an example that makes sure that whoever runs this program
           knows their password:

               $pwd = (getpwuid($<))[1];

               system "stty -echo";
               print "Password: ";
               chomp($word = <STDIN>);
               print "\n";
               system "stty echo";

               if (crypt($word, $pwd) ne $pwd) {
                   die "Sorry...\n";
               } else {
                   print "ok\n";

           Of course, typing in your own password to whoever asks you for it
           is unwise.

           The crypt function is unsuitable for hashing large quantities of
           data, not least of all because you can't get the information back.
           Look at the Digest module for more robust algorithms.

           If using crypt() on a Unicode string (which potentially has
           characters with codepoints above 255), Perl tries to make sense of
           the situation by trying to downgrade (a copy of) the string back to
           an eight-bit byte string before calling crypt() (on that copy).  If
           that works, good.  If not, crypt() dies with "Wide character in
           the first argument is not a filehandle, even though it looks like
           one).  DBNAME is the name of the database (without the .dir or .pag
           extension if any).  If the database does not exist, it is created
           with protection specified by MASK (as modified by the "umask").  If
           your system supports only the older DBM functions, you may make
           only one "dbmopen" call in your program.  In older versions of
           Perl, if your system had neither DBM nor ndbm, calling "dbmopen"
           produced a fatal error; it now falls back to sdbm(3).

           If you don't have write access to the DBM file, you can only read
           hash variables, not set them.  If you want to test whether you can
           write, either use file tests or try setting a dummy hash entry
           inside an "eval" to trap the error.

           Note that functions such as "keys" and "values" may return huge
           lists when used on large DBM files.  You may prefer to use the
           "each" function to iterate over large DBM files.  Example:

               # print out history file offsets
               while (($key,$val) = each %HIST) {
                   print $key, ' = ', unpack('L',$val), "\n";

           See also AnyDBM_File for a more general description of the pros and
           cons of the various dbm approaches, as well as DB_File for a
           particularly rich implementation.

           You can control which DBM library you use by loading that library
           before you call dbmopen():

               use DB_File;
               dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
                   or die "Can't open netscape history file: $!";

       default BLOCK
           Within a "foreach" or a "given", a "default" BLOCK acts like a
           "when" that's always true.  Only available after Perl 5.10, and
           only if the "switch" feature has been requested.  See "when".

       defined EXPR
           Returns a Boolean value telling whether EXPR has a value other than
           the undefined value "undef".  If EXPR is not present, $_ is

           Many operations return "undef" to indicate failure, end of file,
           system error, uninitialized variable, and other exceptional
           conditions.  This function allows you to distinguish "undef" from
           other values.  (A simple Boolean test will not distinguish among
           "undef", zero, the empty string, and "0", which are all equally
           false.)  Note that since "undef" is a valid scalar, its presence
           doesn't necessarily indicate an exceptional condition: "pop"
           allocated.  This behavior may disappear in future versions of Perl.
           You should instead use a simple test for size:

               if (@an_array) { print "has array elements\n" }
               if (%a_hash)   { print "has hash members\n"   }

           When used on a hash element, it tells you whether the value is
           defined, not whether the key exists in the hash.  Use "exists" for
           the latter purpose.


               print if defined $switch{D};
               print "$val\n" while defined($val = pop(@ary));
               die "Can't readlink $sym: $!"
                   unless defined($value = readlink $sym);
               sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
               $debugging = 0 unless defined $debugging;

           Note:  Many folks tend to overuse "defined" and are then surprised
           to discover that the number 0 and "" (the zero-length string) are,
           in fact, defined values.  For example, if you say

               "ab" =~ /a(.*)b/;

           The pattern match succeeds and $1 is defined, although it matched
           "nothing".  It didn't really fail to match anything.  Rather, it
           matched something that happened to be zero characters long.  This
           is all very above-board and honest.  When a function returns an
           undefined value, it's an admission that it couldn't give you an
           honest answer.  So you should use "defined" only when questioning
           the integrity of what you're trying to do.  At other times, a
           simple comparison to 0 or "" is what you want.

           See also "undef", "exists", "ref".

       delete EXPR
           Given an expression that specifies an element or slice of a hash,
           "delete" deletes the specified elements from that hash so that
           exists() on that element no longer returns true.  Setting a hash
           element to the undefined value does not remove its key, but
           deleting it does; see "exists".

           In list context, returns the value or values deleted, or the last
           such element in scalar context.  The return list's length always
           matches that of the argument list: deleting non-existent elements
           returns the undefined value in their corresponding positions.

           delete() may also be used on arrays and array slices, but its
           behavior is less straightforward.  Although exists() will return
           false for deleted entries, deleting array elements never changes
           indices of existing values; use shift() or splice() for that.
           However, if all deleted elements fall at the end of an array, the
           array's size shrinks to the position of the highest element that
           current block at run time.  Until the block exits, elements locally
           deleted temporarily no longer exist.  See "Localized deletion of
           elements of composite types" in perlsub.

               %hash = (foo => 11, bar => 22, baz => 33);
               $scalar = delete $hash{foo};             # $scalar is 11
               $scalar = delete @hash{qw(foo bar)};     # $scalar is 22
               @array  = delete @hash{qw(foo bar baz)}; # @array  is (undef,undef,33)

           The following (inefficiently) deletes all the values of %HASH and

               foreach $key (keys %HASH) {
                   delete $HASH{$key};

               foreach $index (0 .. $#ARRAY) {
                   delete $ARRAY[$index];

           And so do these:

               delete @HASH{keys %HASH};

               delete @ARRAY[0 .. $#ARRAY];

           But both are slower than assigning the empty list or undefining
           %HASH or @ARRAY, which is the customary way to empty out an

               %HASH = ();     # completely empty %HASH
               undef %HASH;    # forget %HASH ever existed

               @ARRAY = ();    # completely empty @ARRAY
               undef @ARRAY;   # forget @ARRAY ever existed

           The EXPR can be arbitrarily complicated provided its final
           operation is an element or slice of an aggregate:

               delete $ref->[$x][$y]{$key};
               delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};

               delete $ref->[$x][$y][$index];
               delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];

       die LIST
           "die" raises an exception. Inside an "eval" the error message is
           stuffed into $@ and the "eval" is terminated with the undefined
           value.  If the exception is outside of all enclosing "eval"s, then
           the uncaught exception prints LIST to "STDERR" and exits with a
           non-zero value. If you need to exit the process with a specific
           exit code, see "exit".

           Equivalent examples:
           to make better sense when the string "at foo line 123" is appended.
           Suppose you are running script "canasta".

               die "/etc/games is no good";
               die "/etc/games is no good, stopped";

           produce, respectively

               /etc/games is no good at canasta line 123.
               /etc/games is no good, stopped at canasta line 123.

           If the output is empty and $@ already contains a value (typically
           from a previous eval) that value is reused after appending
           "\t...propagated".  This is useful for propagating exceptions:

               eval { ... };
               die unless $@ =~ /Expected exception/;

           If the output is empty and $@ contains an object reference that has
           a "PROPAGATE" method, that method will be called with additional
           file and line number parameters.  The return value replaces the
           value in $@;  i.e., as if "$@ = eval { $@->PROPAGATE(__FILE__,
           __LINE__) };" were called.

           If $@ is empty then the string "Died" is used.

           If an uncaught exception results in interpreter exit, the exit code
           is determined from the values of $! and $? with this pseudocode:

               exit $! if $!;              # errno
               exit $? >> 8 if $? >> 8;    # child exit status
               exit 255;                   # last resort

           The intent is to squeeze as much possible information about the
           likely cause into the limited space of the system exit code.
           However, as $! is the value of C's "errno", which can be set by any
           system call, this means that the value of the exit code used by
           "die" can be non-predictable, so should not be relied upon, other
           than to be non-zero.

           You can also call "die" with a reference argument, and if this is
           trapped within an "eval", $@ contains that reference.  This permits
           more elaborate exception handling using objects that maintain
           arbitrary state about the exception.  Such a scheme is sometimes
           preferable to matching particular string values of $@ with regular
           expressions.  Because $@ is a global variable and "eval" may be
           used within object implementations, be careful that analyzing the
           error object doesn't replace the reference in the global variable.
           It's easiest to make a local copy of the reference before any
           manipulations.  Here's an example:

               use Scalar::Util "blessed";

               eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };


           You can arrange for a callback to be run just before the "die" does
           its deed, by setting the $SIG{__DIE__} hook.  The associated
           handler is called with the error text and can change the error
           message, if it sees fit, by calling "die" again.  See "%SIG" in
           perlvar for details on setting %SIG entries, and "eval BLOCK" for
           some examples.  Although this feature was to be run only right
           before your program was to exit, this is not currently so: the
           $SIG{__DIE__} hook is currently called even inside eval()ed
           blocks/strings!  If one wants the hook to do nothing in such
           situations, put

               die @_ if $^S;

           as the first line of the handler (see "$^S" in perlvar).  Because
           this promotes strange action at a distance, this counterintuitive
           behavior may be fixed in a future release.

           See also exit(), warn(), and the Carp module.

       do BLOCK
           Not really a function.  Returns the value of the last command in
           the sequence of commands indicated by BLOCK.  When modified by the
           "while" or "until" loop modifier, executes the BLOCK once before
           testing the loop condition. (On other statements the loop modifiers
           test the conditional first.)

           "do BLOCK" does not count as a loop, so the loop control statements
           "next", "last", or "redo" cannot be used to leave or restart the
           block.  See perlsyn for alternative strategies.

           This form of subroutine call is deprecated.  SUBROUTINE can be a
           bareword, a scalar variable or a subroutine beginning with "&".

       do EXPR
           Uses the value of EXPR as a filename and executes the contents of
           the file as a Perl script.

               do '';

           is just like

               eval `cat`;

           except that it's more efficient and concise, keeps track of the
           current filename for error messages, searches the @INC directories,
           and updates %INC if the file is found.  See "@INC" in perlvar and
           "%INC" in perlvar for these variables.  It also differs in that
           code evaluated with "do FILENAME" cannot see lexicals in the
           enclosing scope; "eval STRING" does.  It's the same, however, in
           that it does reparse the file every time you call it, so you
           probably don't want to do this inside a loop.
           You might like to use "do" to read in a program configuration file.
           Manual error checking can be done this way:

               # read in config files: system first, then user
               for $file ("/share/prog/defaults.rc",
                   unless ($return = do $file) {
                       warn "couldn't parse $file: $@" if $@;
                       warn "couldn't do $file: $!"    unless defined $return;
                       warn "couldn't run $file"       unless $return;

       dump LABEL
           This function causes an immediate core dump.  See also the -u
           command-line switch in perlrun, which does the same thing.
           Primarily this is so that you can use the undump program (not
           supplied) to turn your core dump into an executable binary after
           having initialized all your variables at the beginning of the
           program.  When the new binary is executed it will begin by
           executing a "goto LABEL" (with all the restrictions that "goto"
           suffers).  Think of it as a goto with an intervening core dump and
           reincarnation.  If "LABEL" is omitted, restarts the program from
           the top.

           WARNING: Any files opened at the time of the dump will not be open
           any more when the program is reincarnated, with possible resulting
           confusion by Perl.

           This function is now largely obsolete, mostly because it's very
           hard to convert a core file into an executable. That's why you
           should now invoke it as "CORE::dump()", if you don't want to be
           warned against a possible typo.

       each HASH
       each ARRAY
       each EXPR
           When called in list context, returns a 2-element list consisting of
           the key and value for the next element of a hash, or the index and
           value for the next element of an array, so that you can iterate
           over it.  When called in scalar context, returns only the key (not
           the value) in a hash, or the index in an array.

           Hash entries are returned in an apparently random order.  The
           actual random order is subject to change in future versions of
           Perl, but it is guaranteed to be in the same order as either the
           "keys" or "values" function would produce on the same (unmodified)
           hash.  Since Perl 5.8.2 the ordering can be different even between
           different runs of Perl for security reasons (see "Algorithmic
           Complexity Attacks" in perlsec).

           After "each" has returned all entries from the hash or array, the
                   while (($key, $value) = each %hash) {
                     print $key, "\n";
                     delete $hash{$key};   # This is safe

           This prints out your environment like the printenv(1) program, but
           in a different order:

               while (($key,$value) = each %ENV) {
                   print "$key=$value\n";

           Starting with Perl 5.14, "each" can take a scalar EXPR, which must
           hold reference to an unblessed hash or array.  The argument will be
           dereferenced automatically.  This aspect of "each" is considered
           highly experimental.  The exact behaviour may change in a future
           version of Perl.

               while (($key,$value) = each $hashref) { ... }

           See also "keys", "values", and "sort".

       eof FILEHANDLE
       eof ()
       eof Returns 1 if the next read on FILEHANDLE will return end of file or
           if FILEHANDLE is not open.  FILEHANDLE may be an expression whose
           value gives the real filehandle.  (Note that this function actually
           reads a character and then "ungetc"s it, so isn't useful in an
           interactive context.)  Do not read from a terminal file (or call
           "eof(FILEHANDLE)" on it) after end-of-file is reached.  File types
           such as terminals may lose the end-of-file condition if you do.

           An "eof" without an argument uses the last file read.  Using
           "eof()" with empty parentheses is different.  It refers to the
           pseudo file formed from the files listed on the command line and
           accessed via the "<>" operator.  Since "<>" isn't explicitly
           opened, as a normal filehandle is, an "eof()" before "<>" has been
           used will cause @ARGV to be examined to determine if input is
           available.   Similarly, an "eof()" after "<>" has returned end-of-
           file will assume you are processing another @ARGV list, and if you
           haven't set @ARGV, will read input from "STDIN"; see "I/O
           Operators" in perlop.

           In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to detect
           the end of each file, whereas "eof()" will detect the end of the
           very last file only.  Examples:

               # reset line numbering on each input file
               while (<>) {
                   next if /^\s*#/;  # skip comments
                   print "$.\t$_";
               } continue {
                   close ARGV if eof;  # Not eof()!

           data or encounter an error.

       eval EXPR
       eval BLOCK
           In the first form, the return value of EXPR is parsed and executed
           as if it were a little Perl program.  The value of the expression
           (which is itself determined within scalar context) is first parsed,
           and if there were no errors, executed in the lexical context of the
           current Perl program, so that any variable settings or subroutine
           and format definitions remain afterwards.  Note that the value is
           parsed every time the "eval" executes.  If EXPR is omitted,
           evaluates $_.  This form is typically used to delay parsing and
           subsequent execution of the text of EXPR until run time.

           In the second form, the code within the BLOCK is parsed only
           once--at the same time the code surrounding the "eval" itself was
           parsed--and executed within the context of the current Perl
           program.  This form is typically used to trap exceptions more
           efficiently than the first (see below), while also providing the
           benefit of checking the code within BLOCK at compile time.

           The final semicolon, if any, may be omitted from the value of EXPR
           or within the BLOCK.

           In both forms, the value returned is the value of the last
           expression evaluated inside the mini-program; a return statement
           may be also used, just as with subroutines.  The expression
           providing the return value is evaluated in void, scalar, or list
           context, depending on the context of the "eval" itself.  See
           "wantarray" for more on how the evaluation context can be

           If there is a syntax error or runtime error, or a "die" statement
           is executed, "eval" returns "undef" in scalar context or an empty
           list--or, for syntax errors, a list containing a single undefined
           value--in list context, and $@ is set to the error message.  The
           discrepancy in the return values in list context is considered a
           bug by some, and will probably be fixed in a future release.  If
           there was no error, $@ is guaranteed to be the empty string.
           Beware that using "eval" neither silences Perl from printing
           warnings to STDERR, nor does it stuff the text of warning messages
           into $@.  To do either of those, you have to use the $SIG{__WARN__}
           facility, or turn off warnings inside the BLOCK or EXPR using
           "no warnings 'all'".  See "warn", perlvar, warnings and

           Note that, because "eval" traps otherwise-fatal errors, it is
           useful for determining whether a particular feature (such as
           "socket" or "symlink") is implemented.  It is also Perl's
           exception-trapping mechanism, where the die operator is used to
           raise exceptions.

           If you want to trap errors when loading an XS module, some problems
               eval '$answer = $a / $b'; warn $@ if $@;

               # a compile-time error
               eval { $answer = }; # WRONG

               # a run-time error
               eval '$answer =';   # sets $@

           Using the "eval{}" form as an exception trap in libraries does have
           some issues.  Due to the current arguably broken state of "__DIE__"
           hooks, you may wish not to trigger any "__DIE__" hooks that user
           code may have installed.  You can use the "local $SIG{__DIE__}"
           construct for this purpose, as this example shows:

               # a private exception trap for divide-by-zero
               eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
               warn $@ if $@;

           This is especially significant, given that "__DIE__" hooks can call
           "die" again, which has the effect of changing their error messages:

               # __DIE__ hooks may modify error messages
                  local $SIG{'__DIE__'} =
                         sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
                  eval { die "foo lives here" };
                  print $@ if $@;                # prints "bar lives here"

           Because this promotes action at a distance, this counterintuitive
           behavior may be fixed in a future release.

           With an "eval", you should be especially careful to remember what's
           being looked at when:

               eval $x;        # CASE 1
               eval "$x";      # CASE 2

               eval '$x';      # CASE 3
               eval { $x };    # CASE 4

               eval "\$$x++";  # CASE 5
               $$x++;          # CASE 6

           Cases 1 and 2 above behave identically: they run the code contained
           in the variable $x.  (Although case 2 has misleading double quotes
           making the reader wonder what else might be happening (nothing
           is).)  Cases 3 and 4 likewise behave in the same way: they run the
           code '$x', which does nothing but return the value of $x.  (Case 4
           is preferred for purely visual reasons, but it also has the
           advantage of compiling at compile-time instead of at run-time.)
           Case 5 is a place where normally you would like to use double
           quotes, except that in this particular situation, you can just use
           symbolic references instead, as in case 6.
                     # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
                     $@ =~ /nefarious/ and $e = $@;
                  die $e if defined $e

           "eval BLOCK" does not count as a loop, so the loop control
           statements "next", "last", or "redo" cannot be used to leave or
           restart the block.

           An "eval ''" executed within the "DB" package doesn't see the usual
           surrounding lexical scope, but rather the scope of the first non-DB
           piece of code that called it. You don't normally need to worry
           about this unless you are writing a Perl debugger.

       exec LIST
       exec PROGRAM LIST
           The "exec" function executes a system command and never returns;
           use "system" instead of "exec" if you want it to return.  It fails
           and returns false only if the command does not exist and it is
           executed directly instead of via your system's command shell (see

           Since it's a common mistake to use "exec" instead of "system", Perl
           warns you if there is a following statement that isn't "die",
           "warn", or "exit" (if "-w" is set--but you always do that, right?).
           If you really want to follow an "exec" with some other statement,
           you can use one of these styles to avoid the warning:

               exec ('foo')   or print STDERR "couldn't exec foo: $!";
               { exec ('foo') }; print STDERR "couldn't exec foo: $!";

           If there is more than one argument in LIST, or if LIST is an array
           with more than one value, calls execvp(3) with the arguments in
           LIST.  If there is only one scalar argument or an array with one
           element in it, the argument is checked for shell metacharacters,
           and if there are any, the entire argument is passed to the system's
           command shell for parsing (this is "/bin/sh -c" on Unix platforms,
           but varies on other platforms).  If there are no shell
           metacharacters in the argument, it is split into words and passed
           directly to "execvp", which is more efficient.  Examples:

               exec '/bin/echo', 'Your arguments are: ', @ARGV;
               exec "sort $outfile | uniq";

           If you don't really want to execute the first argument, but want to
           lie to the program you are executing about its own name, you can
           specify the program you actually want to run as an "indirect
           object" (without a comma) in front of the LIST.  (This always
           forces interpretation of the LIST as a multivalued list, even if
           there is only a single scalar in the list.)  Example:

               $shell = '/bin/csh';
               exec $shell '-sh';    # pretend it's a login shell

           list had just one argument.  That way you're safe from the shell
           expanding wildcards or splitting up words with whitespace in them.

               @args = ( "echo surprise" );

               exec @args;               # subject to shell escapes
                                           # if @args == 1
               exec { $args[0] } @args;  # safe even with one-arg list

           The first version, the one without the indirect object, ran the
           echo program, passing it "surprise" an argument.  The second
           version didn't; it tried to run a program named "echo surprise",
           didn't find it, and set $? to a non-zero value indicating failure.

           Beginning with v5.6.0, Perl attempts to flush all files opened for
           output before the exec, but this may not be supported on some
           platforms (see perlport).  To be safe, you may need to set $|
           ($AUTOFLUSH in English) or call the "autoflush()" method of
           "IO::Handle" on any open handles to avoid lost output.

           Note that "exec" will not call your "END" blocks, nor will it
           invoke "DESTROY" methods on your objects.

       exists EXPR
           Given an expression that specifies an element of a hash, returns
           true if the specified element in the hash has ever been
           initialized, even if the corresponding value is undefined.

               print "Exists\n"    if exists $hash{$key};
               print "Defined\n"   if defined $hash{$key};
               print "True\n"      if $hash{$key};

           exists may also be called on array elements, but its behavior is
           much less obvious and is strongly tied to the use of "delete" on
           arrays.  Be aware that calling exists on array values is deprecated
           and likely to be removed in a future version of Perl.

               print "Exists\n"    if exists $array[$index];
               print "Defined\n"   if defined $array[$index];
               print "True\n"      if $array[$index];

           A hash or array element can be true only if it's defined and
           defined only if it exists, but the reverse doesn't necessarily hold

           Given an expression that specifies the name of a subroutine,
           returns true if the specified subroutine has ever been declared,
           even if it is undefined.  Mentioning a subroutine name for exists
           or defined does not count as declaring it.  Note that a subroutine
           that does not exist may still be callable: its package may have an
           "AUTOLOAD" method that makes it spring into existence the first
           time that it is called; see perlsub.

               print "Exists\n"  if exists &subroutine;

           Although the mostly deeply nested array or hash will not spring
           into existence just because its existence was tested, any
           intervening ones will.  Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}"
           will spring into existence due to the existence test for the $key
           element above.  This happens anywhere the arrow operator is used,
           including even here:

               undef $ref;
               if (exists $ref->{"Some key"})    { }
               print $ref;  # prints HASH(0x80d3d5c)

           This surprising autovivification in what does not at first--or even
           second--glance appear to be an lvalue context may be fixed in a
           future release.

           Use of a subroutine call, rather than a subroutine name, as an
           argument to exists() is an error.

               exists &sub;    # OK
               exists &sub();  # Error

       exit EXPR
           Evaluates EXPR and exits immediately with that value.    Example:

               $ans = <STDIN>;
               exit 0 if $ans =~ /^[Xx]/;

           See also "die".  If EXPR is omitted, exits with 0 status.  The only
           universally recognized values for EXPR are 0 for success and 1 for
           error; other values are subject to interpretation depending on the
           environment in which the Perl program is running.  For example,
           exiting 69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter
           will cause the mailer to return the item undelivered, but that's
           not true everywhere.

           Don't use "exit" to abort a subroutine if there's any chance that
           someone might want to trap whatever error happened.  Use "die"
           instead, which can be trapped by an "eval".

           The exit() function does not always exit immediately.  It calls any
           defined "END" routines first, but these "END" routines may not
           themselves abort the exit.  Likewise any object destructors that
           need to be called are called before the real exit.  "END" routines
           and destructors can change the exit status by modifying $?. If this
           is a problem, you can call "POSIX:_exit($status)" to avoid END and
           destructor processing.  See perlmod for details.

       exp EXPR
       exp Returns e (the natural logarithm base) to the power of EXPR.  If
           EXPR is omitted, gives "exp($_)".

           Like "ioctl", it maps a 0 return from the system call into "0 but
           true" in Perl.  This string is true in boolean context and 0 in
           numeric context.  It is also exempt from the normal -w warnings on
           improper numeric conversions.

           Note that "fcntl" raises an exception if used on a machine that
           doesn't implement fcntl(2).  See the Fcntl module or your fcntl(2)
           manpage to learn what functions are available on your system.

           Here's an example of setting a filehandle named "REMOTE" to be non-
           blocking at the system level.  You'll have to negotiate $| on your
           own, though.

               use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);

               $flags = fcntl(REMOTE, F_GETFL, 0)
                           or die "Can't get flags for the socket: $!\n";

               $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
                           or die "Can't set flags for the socket: $!\n";

       fileno FILEHANDLE
           Returns the file descriptor for a filehandle, or undefined if the
           filehandle is not open.  If there is no real file descriptor at the
           OS level, as can happen with filehandles connected to memory
           objects via "open" with a reference for the third argument, -1 is

           This is mainly useful for constructing bitmaps for "select" and
           low-level POSIX tty-handling operations.  If FILEHANDLE is an
           expression, the value is taken as an indirect filehandle, generally
           its name.

           You can use this to find out whether two handles refer to the same
           underlying descriptor:

               if (fileno(THIS) == fileno(THAT)) {
                   print "THIS and THAT are dups\n";

           Calls flock(2), or an emulation of it, on FILEHANDLE.  Returns true
           for success, false on failure.  Produces a fatal error if used on a
           machine that doesn't implement flock(2), fcntl(2) locking, or
           lockf(3).  "flock" is Perl's portable file-locking interface,
           although it locks entire files only, not records.

           Two potentially non-obvious but traditional "flock" semantics are
           that it waits indefinitely until the lock is granted, and that its
           locks are merely advisory.  Such discretionary locks are more
           flexible, but offer fewer guarantees.  This means that programs
           that do not also use "flock" may modify files locked with "flock".
           See perlport, your port's specific documentation, and your system-
           specific local manpages for details.  It's best to assume
           is bitwise-or'ed with LOCK_SH or LOCK_EX, then "flock" returns
           immediately rather than blocking waiting for the lock; check the
           return status to see if you got it.

           To avoid the possibility of miscoordination, Perl now flushes
           FILEHANDLE before locking or unlocking it.

           Note that the emulation built with lockf(3) doesn't provide shared
           locks, and it requires that FILEHANDLE be open with write intent.
           These are the semantics that lockf(3) implements.  Most if not all
           systems implement lockf(3) in terms of fcntl(2) locking, though, so
           the differing semantics shouldn't bite too many people.

           Note that the fcntl(2) emulation of flock(3) requires that
           FILEHANDLE be open with read intent to use LOCK_SH and requires
           that it be open with write intent to use LOCK_EX.

           Note also that some versions of "flock" cannot lock things over the
           network; you would need to use the more system-specific "fcntl" for
           that.  If you like you can force Perl to ignore your system's
           flock(2) function, and so provide its own fcntl(2)-based emulation,
           by passing the switch "-Ud_flock" to the Configure program when you
           configure and build a new Perl.

           Here's a mailbox appender for BSD systems.

               use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants

               sub lock {
                   my ($fh) = @_;
                   flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";

                   # and, in case someone appended while we were waiting...
                   seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";

               sub unlock {
                   my ($fh) = @_;
                   flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";

               open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
                   or die "Can't open mailbox: $!";

               print $mbox $msg,"\n\n";

           On systems that support a real flock(2), locks are inherited across
           fork() calls, whereas those that must resort to the more capricious
           fcntl(2) function lose their locks, making it seriously harder to
           write servers.

           See also DB_File for other flock() examples.
           Beginning with v5.6.0, Perl attempts to flush all files opened for
           output before forking the child process, but this may not be
           supported on some platforms (see perlport).  To be safe, you may
           need to set $| ($AUTOFLUSH in English) or call the "autoflush()"
           method of "IO::Handle" on any open handles to avoid duplicate

           If you "fork" without ever waiting on your children, you will
           accumulate zombies.  On some systems, you can avoid this by setting
           $SIG{CHLD} to "IGNORE".  See also perlipc for more examples of
           forking and reaping moribund children.

           Note that if your forked child inherits system file descriptors
           like STDIN and STDOUT that are actually connected by a pipe or
           socket, even if you exit, then the remote server (such as, say, a
           CGI script or a backgrounded job launched from a remote shell)
           won't think you're done.  You should reopen those to /dev/null if
           it's any issue.

           Declare a picture format for use by the "write" function.  For

               format Something =
                   Test: @<<<<<<<< @||||| @>>>>>
                         $str,     $%,    '$' . int($num)

               $str = "widget";
               $num = $cost/$quantity;
               $~ = 'Something';

           See perlform for many details and examples.

       formline PICTURE,LIST
           This is an internal function used by "format"s, though you may call
           it, too.  It formats (see perlform) a list of values according to
           the contents of PICTURE, placing the output into the format output
           accumulator, $^A (or $ACCUMULATOR in English).  Eventually, when a
           "write" is done, the contents of $^A are written to some
           filehandle.  You could also read $^A and then set $^A back to "".
           Note that a format typically does one "formline" per line of form,
           but the "formline" function itself doesn't care how many newlines
           are embedded in the PICTURE.  This means that the "~" and "~~"
           tokens treat the entire PICTURE as a single line.  You may
           therefore need to use multiple formlines to implement a single
           record format, just like the "format" compiler.

           Be careful if you put double quotes around the picture, because an
           "@" character may be taken to mean the beginning of an array name.
           "formline" always returns true.  See perlform for other examples.

           If you are trying to use this instead of "write" to capture the
               if ($BSD_STYLE) {
                   system "stty cbreak </dev/tty >/dev/tty 2>&1";
               else {
                   system "stty", '-icanon', 'eol', "\001";

               $key = getc(STDIN);

               if ($BSD_STYLE) {
                   system "stty -cbreak </dev/tty >/dev/tty 2>&1";
               else {
                   system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
               print "\n";

           Determination of whether $BSD_STYLE should be set is left as an
           exercise to the reader.

           The "POSIX::getattr" function can do this more portably on systems
           purporting POSIX compliance.  See also the "Term::ReadKey" module
           from your nearest CPAN site; details on CPAN can be found under
           "CPAN" in perlmodlib.

           This implements the C library function of the same name, which on
           most systems returns the current login from /etc/utmp, if any.  If
           it returns the empty string, use "getpwuid".

               $login = getlogin || getpwuid($<) || "Kilroy";

           Do not consider "getlogin" for authentication: it is not as secure
           as "getpwuid".

       getpeername SOCKET
           Returns the packed sockaddr address of the other end of the SOCKET

               use Socket;
               $hersockaddr    = getpeername(SOCK);
               ($port, $iaddr) = sockaddr_in($hersockaddr);
               $herhostname    = gethostbyaddr($iaddr, AF_INET);
               $herstraddr     = inet_ntoa($iaddr);

       getpgrp PID
           Returns the current process group for the specified PID.  Use a PID
           of 0 to get the current process group for the current process.
           Will raise an exception if used on a machine that doesn't implement
           getpgrp(2).  If PID is omitted, returns the process group of the
           current process.  Note that the POSIX version of "getpgrp" does not
           accept a PID argument, so only "PID==0" is truly portable.

           on a machine that doesn't implement getpriority(2).

       getpwnam NAME
       getgrnam NAME
       gethostbyname NAME
       getnetbyname NAME
       getprotobyname NAME
       getpwuid UID
       getgrgid GID
       getservbyname NAME,PROTO
       gethostbyaddr ADDR,ADDRTYPE
       getnetbyaddr ADDR,ADDRTYPE
       getprotobynumber NUMBER
       getservbyport PORT,PROTO
       sethostent STAYOPEN
       setnetent STAYOPEN
       setprotoent STAYOPEN
       setservent STAYOPEN
           These routines are the same as their counterparts in the system C
           library.  In list context, the return values from the various get
           routines are as follows:

                  $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
               ($name,$passwd,$gid,$members) = getgr*
               ($name,$aliases,$addrtype,$length,@addrs) = gethost*
               ($name,$aliases,$addrtype,$net) = getnet*
               ($name,$aliases,$proto) = getproto*
               ($name,$aliases,$port,$proto) = getserv*

           (If the entry doesn't exist you get an empty list.)

           The exact meaning of the $gcos field varies but it usually contains
           the real name of the user (as opposed to the login name) and other
           information pertaining to the user.  Beware, however, that in many
           system users are able to change this information and therefore it
           cannot be trusted and therefore the $gcos is tainted (see perlsec).
           The $passwd and $shell, user's encrypted password and login shell,
           are also tainted, for the same reason.

           In getpw*() the fields $quota, $comment, and $expire are special in
           that they are unsupported on many systems.  If the $quota is
           unsupported, it is an empty scalar.  If it is supported, it usually
           encodes the disk quota.  If the $comment field is unsupported, it
           is an empty scalar.  If it is supported it usually encodes some
           administrative comment about the user.  In some systems the $quota
           field may be $change or $age, fields that have to do with password
           aging.  In some systems the $comment field may be $class.  The
           $expire field, if present, encodes the expiration period of the
           account or the password.  For the availability and the exact
           meaning of these fields in your system, please consult getpwnam(3)
           and your system's pwd.h file.  You can also find out from within
           Perl what your $quota and $comment fields mean and whether you have
           the $expire field by using the "Config" module and the values
           "d_pwquota", "d_pwage", "d_pwchange", "d_pwcomment", and
           "d_pwexpire".  Shadow password files are supported only if your
           vendor has implemented them in the intuitive fashion that calling
           the regular C library routines gets the shadow versions if you're
           running under privilege or if there exists the shadow(3) functions
           as found in System V (this includes Solaris and Linux).  Those
           systems that implement a proprietary shadow password facility are
           unlikely to be supported.

           The $members value returned by getgr*() is a space-separated list
           of the login names of the members of the group.

           For the gethost*() functions, if the "h_errno" variable is
           supported in C, it will be returned to you via $? if the function
           call fails.  The @addrs value returned by a successful call is a
           list of raw addresses returned by the corresponding library call.
           In the Internet domain, each address is four bytes long; you can
           unpack it by saying something like:

               ($a,$b,$c,$d) = unpack('W4',$addr[0]);

           The Socket library makes this slightly easier:

               use Socket;
               $iaddr = inet_aton("127.1"); # or whatever address
               $name  = gethostbyaddr($iaddr, AF_INET);

               # or going the other way
               $straddr = inet_ntoa($iaddr);

           In the opposite way, to resolve a hostname to the IP address you
           can write this:

               use Socket;
               $packed_ip = gethostbyname("");
               if (defined $packed_ip) {
                   $ip_address = inet_ntoa($packed_ip);

              use User::pwent;
              $is_his = (stat($filename)->uid == pwent($whoever)->uid);

           Even though it looks as though they're the same method calls (uid),
           they aren't, because a "File::stat" object is different from a
           "User::pwent" object.

       getsockname SOCKET
           Returns the packed sockaddr address of this end of the SOCKET
           connection, in case you don't know the address because you have
           several different IPs that the connection might have come in on.

               use Socket;
               $mysockaddr = getsockname(SOCK);
               ($port, $myaddr) = sockaddr_in($mysockaddr);
               printf "Connect to %s [%s]\n",
                  scalar gethostbyaddr($myaddr, AF_INET),

       getsockopt SOCKET,LEVEL,OPTNAME
           Queries the option named OPTNAME associated with SOCKET at a given
           LEVEL.  Options may exist at multiple protocol levels depending on
           the socket type, but at least the uppermost socket level SOL_SOCKET
           (defined in the "Socket" module) will exist. To query options at
           another level the protocol number of the appropriate protocol
           controlling the option should be supplied. For example, to indicate
           that an option is to be interpreted by the TCP protocol, LEVEL
           should be set to the protocol number of TCP, which you can get
           using "getprotobyname".

           The function returns a packed string representing the requested
           socket option, or "undef" on error, with the reason for the error
           placed in $!. Just what is in the packed string depends on LEVEL
           and OPTNAME; consult getsockopt(2) for details.  A common case is
           that the option is an integer, in which case the result is a packed
           integer, which you can decode using "unpack" with the "i" (or "I")

           Here's an example to test whether Nagle's algorithm is enabled on a

               use Socket qw(:all);

               defined(my $tcp = getprotobyname("tcp"))
                   or die "Could not determine the protocol number for tcp";
               # my $tcp = IPPROTO_TCP; # Alternative
               my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
                   or die "getsockopt TCP_NODELAY: $!";
               my $nodelay = unpack("I", $packed);
               print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";

       given EXPR BLOCK
       given BLOCK
           "given" is analogous to the "switch" keyword in other languages.
                       print "I don't like anything"

           See "Switch statements" in perlsyn for detailed information.

       glob EXPR
           In list context, returns a (possibly empty) list of filename
           expansions on the value of EXPR such as the standard Unix shell
           /bin/csh would do. In scalar context, glob iterates through such
           filename expansions, returning undef when the list is exhausted.
           This is the internal function implementing the "<*.c>" operator,
           but you can use it directly. If EXPR is omitted, $_ is used.  The
           "<*.c>" operator is discussed in more detail in "I/O Operators" in

           Note that "glob" splits its arguments on whitespace and treats each
           segment as separate pattern.  As such, "glob("*.c *.h")" matches
           all files with a .c or .h extension.  The expression "glob(".* *")"
           matches all files in the current working directory.

           If non-empty braces are the only wildcard characters used in the
           "glob", no filenames are matched, but potentially many strings are
           returned.  For example, this produces nine strings, one for each
           pairing of fruits and colors:

               @many =  glob "{apple,tomato,cherry}={green,yellow,red}";

           Beginning with v5.6.0, this operator is implemented using the
           standard "File::Glob" extension.  See File::Glob for details,
           including "bsd_glob" which does not treat whitespace as a pattern

       gmtime EXPR
           Works just like localtime but the returned values are localized for
           the standard Greenwich time zone.

           Note: When called in list context, $isdst, the last value returned
           by gmtime, is always 0.  There is no Daylight Saving Time in GMT.

           See "gmtime" in perlport for portability concerns.

       goto LABEL
       goto EXPR
       goto &NAME
           The "goto-LABEL" form finds the statement labeled with LABEL and
           resumes execution there. It can't be used to get out of a block or
           subroutine given to "sort".  It can be used to go almost anywhere
           else within the dynamic scope, including out of subroutines, but
           it's usually better to use some other construct such as "last" or
           "die".  The author of Perl has never felt the need to use this form
           of "goto" (in Perl, that is; C is another matter).  (The difference
           like a function" rule. A pair of parentheses following it does not
           (necessarily) delimit its argument. "goto("NE")."XT"" is equivalent
           to "goto NEXT".

           Use of "goto-LABEL" or "goto-EXPR" to jump into a construct is
           deprecated and will issue a warning.  Even then, it may not be used
           to go into any construct that requires initialization, such as a
           subroutine or a "foreach" loop.  It also can't be used to go into a
           construct that is optimized away.

           The "goto-&NAME" form is quite different from the other forms of
           "goto".  In fact, it isn't a goto in the normal sense at all, and
           doesn't have the stigma associated with other gotos.  Instead, it
           exits the current subroutine (losing any changes set by local())
           and immediately calls in its place the named subroutine using the
           current value of @_.  This is used by "AUTOLOAD" subroutines that
           wish to load another subroutine and then pretend that the other
           subroutine had been called in the first place (except that any
           modifications to @_ in the current subroutine are propagated to the
           other subroutine.)  After the "goto", not even "caller" will be
           able to tell that this routine was called first.

           NAME needn't be the name of a subroutine; it can be a scalar
           variable containing a code reference or a block that evaluates to a
           code reference.

       grep BLOCK LIST
       grep EXPR,LIST
           This is similar in spirit to, but not the same as, grep(1) and its
           relatives.  In particular, it is not limited to using regular

           Evaluates the BLOCK or EXPR for each element of LIST (locally
           setting $_ to each element) and returns the list value consisting
           of those elements for which the expression evaluated to true.  In
           scalar context, returns the number of times the expression was

               @foo = grep(!/^#/, @bar);    # weed out comments

           or equivalently,

               @foo = grep {!/^#/} @bar;    # weed out comments

           Note that $_ is an alias to the list value, so it can be used to
           modify the elements of the LIST.  While this is useful and
           supported, it can cause bizarre results if the elements of LIST are
           not variables.  Similarly, grep returns aliases into the original
           list, much as a for loop's index variable aliases the list
           elements.  That is, modifying an element of a list returned by grep
           (for example, in a "foreach", "map" or another "grep") actually
           modifies the element in the original list.  This is usually
           something to be avoided when writing clear code.

           or "0b", see "oct".)  If EXPR is omitted, uses $_.

               print hex '0xAf'; # prints '175'
               print hex 'aF';   # same

           Hex strings may only represent integers.  Strings that would cause
           integer overflow trigger a warning.  Leading whitespace is not
           stripped, unlike oct(). To present something as hex, look into
           "printf", "sprintf", and "unpack".

       import LIST
           There is no builtin "import" function.  It is just an ordinary
           method (subroutine) defined (or inherited) by modules that wish to
           export names to another module.  The "use" function calls the
           "import" method for the package used.  See also "use", perlmod, and

       index STR,SUBSTR
           The index function searches for one string within another, but
           without the wildcard-like behavior of a full regular-expression
           pattern match.  It returns the position of the first occurrence of
           SUBSTR in STR at or after POSITION.  If POSITION is omitted, starts
           searching from the beginning of the string.  POSITION before the
           beginning of the string or after its end is treated as if it were
           the beginning or the end, respectively.  POSITION and the return
           value are based at 0 (or whatever you've set the $[ variable
           to--but don't do that).  If the substring is not found, "index"
           returns one less than the base, ordinarily "-1".

       int EXPR
       int Returns the integer portion of EXPR.  If EXPR is omitted, uses $_.
           You should not use this function for rounding: one because it
           truncates towards 0, and two because machine representations of
           floating-point numbers can sometimes produce counterintuitive
           results.  For example, "int(-6.725/0.025)" produces -268 rather
           than the correct -269; that's because it's really more like
           -268.99999999999994315658 instead.  Usually, the "sprintf",
           "printf", or the "POSIX::floor" and "POSIX::ceil" functions will
           serve you better than will int().

           Implements the ioctl(2) function.  You'll probably first have to

               require "sys/";  # probably in $Config{archlib}/sys/

           to get the correct function definitions.  If sys/ doesn't
           exist or doesn't have the correct definitions you'll have to roll
           your own, based on your C header files such as <sys/ioctl.h>.
           (There is a Perl script called h2ph that comes with the Perl kit
           that may help you in this, but it's nontrivial.)  SCALAR will be
           read and/or written depending on the FUNCTION; a C pointer to the
           string value of SCALAR will be passed as the third argument of the

           Thus Perl returns true on success and false on failure, yet you can
           still easily determine the actual value returned by the operating

               $retval = ioctl(...) || -1;
               printf "System returned %d\n", $retval;

           The special string "0 but true" is exempt from -w complaints about
           improper numeric conversions.

       join EXPR,LIST
           Joins the separate strings of LIST into a single string with fields
           separated by the value of EXPR, and returns that new string.

               $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);

           Beware that unlike "split", "join" doesn't take a pattern as its
           first argument.  Compare "split".

       keys HASH
       keys ARRAY
       keys EXPR
           Returns a list consisting of all the keys of the named hash, or the
           indices of an array. (In scalar context, returns the number of keys
           or indices.)

           The keys of a hash are returned in an apparently random order.  The
           actual random order is subject to change in future versions of
           Perl, but it is guaranteed to be the same order as either the
           "values" or "each" function produces (given that the hash has not
           been modified).  Since Perl 5.8.1 the ordering can be different
           even between different runs of Perl for security reasons (see
           "Algorithmic Complexity Attacks" in perlsec).

           As a side effect, calling keys() resets the internal interator of
           the HASH or ARRAY (see "each").  In particular, calling keys() in
           void context resets the iterator with no other overhead.

           Here is yet another way to print your environment:

               @keys = keys %ENV;
               @values = values %ENV;
               while (@keys) {
                   print pop(@keys), '=', pop(@values), "\n";

           or how about sorted by key:

               foreach $key (sort(keys %ENV)) {
                   print $key, '=', $ENV{$key}, "\n";

           buckets allocated for the given hash.  This can gain you a measure
           of efficiency if you know the hash is going to get big.  (This is
           similar to pre-extending an array by assigning a larger number to
           $#array.)  If you say

               keys %hash = 200;

           then %hash will have at least 200 buckets allocated for it--256 of
           them, in fact, since it rounds up to the next power of two.  These
           buckets will be retained even if you do "%hash = ()", use "undef
           %hash" if you want to free the storage while %hash is still in
           scope.  You can't shrink the number of buckets allocated for the
           hash using "keys" in this way (but you needn't worry about doing
           this by accident, as trying has no effect). "keys @array" in an
           lvalue context is a syntax error.

           Starting with Perl 5.14, "keys" can take a scalar EXPR, which must
           contain a reference to an unblessed hash or array.  The argument
           will be dereferenced automatically.  This aspect of "keys" is
           considered highly experimental.  The exact behaviour may change in
           a future version of Perl.

               for (keys $hashref) { ... }
               for (keys $obj->get_arrayref) { ... }

           See also "each", "values", and "sort".

       kill SIGNAL, LIST
           Sends a signal to a list of processes.  Returns the number of
           processes successfully signaled (which is not necessarily the same
           as the number actually killed).

               $cnt = kill 1, $child1, $child2;
               kill 9, @goners;

           If SIGNAL is zero, no signal is sent to the process, but "kill"
           checks whether it's possible to send a signal to it (that means, to
           be brief, that the process is owned by the same user, or we are the
           super-user).  This is useful to check that a child process is still
           alive (even if only as a zombie) and hasn't changed its UID.  See
           perlport for notes on the portability of this construct.

           Unlike in the shell, if SIGNAL is negative, it kills process groups
           instead of processes. That means you usually want to use positive
           not negative signals.  You may also use a signal name in quotes.

           The behavior of kill when a PROCESS number is zero or negative
           depends on the operating system.  For example, on POSIX-conforming
           systems, zero will signal the current process group and -1 will
           signal all processes.

           See "Signals" in perlipc for more details.

       last LABEL
           "eval {}", "sub {}", or "do {}", and should not be used to exit a
           grep() or map() operation.

           Note that a block by itself is semantically identical to a loop
           that executes once.  Thus "last" can be used to effect an early
           exit out of such a block.

           See also "continue" for an illustration of how "last", "next", and
           "redo" work.

       lc EXPR
       lc  Returns a lowercased version of EXPR.  This is the internal
           function implementing the "\L" escape in double-quoted strings.

           If EXPR is omitted, uses $_.

           What gets returned depends on several factors:

           If "use bytes" is in effect:
               On EBCDIC platforms
                   The results are what the C language system call "tolower()"

               On ASCII platforms
                   The results follow ASCII semantics.  Only characters "A-Z"
                   change, to "a-z" respectively.

           Otherwise, If EXPR has the UTF8 flag set
               If the current package has a subroutine named "ToLower", it
               will be used to change the case (See "User-Defined Case
               Mappings (for serious hackers only)" in perlunicode.)
               Otherwise Unicode semantics are used for the case change.

           Otherwise, if "use locale" is in effect
               Respects current LC_CTYPE locale.  See perllocale.

           Otherwise, if "use feature 'unicode_strings'" is in effect:
               Unicode semantics are used for the case change.  Any subroutine
               named "ToLower" will be ignored.

               On EBCDIC platforms
                   The results are what the C language system call "tolower()"

               On ASCII platforms
                   ASCII semantics are used for the case change.  The
                   lowercase of any character outside the ASCII range is the
                   character itself.

       lcfirst EXPR
           Returns the value of EXPR with the first character lowercased.
           This is the internal function implementing the "\l" escape in

           This function cannot be used on an entire array or hash to find out
           how many elements these have.  For that, use "scalar @array" and
           "scalar keys %hash", respectively.

           Like all Perl character operations, length() normally deals in
           logical characters, not physical bytes.  For how many bytes a
           string encoded as UTF-8 would take up, use
           "length(Encode::encode_utf8(EXPR))" (you'll have to "use Encode"
           first).  See Encode and perlunicode.

           Creates a new filename linked to the old filename.  Returns true
           for success, false otherwise.

       listen SOCKET,QUEUESIZE
           Does the same thing that the listen(2) system call does.  Returns
           true if it succeeded, false otherwise.  See the example in
           "Sockets: Client/Server Communication" in perlipc.

       local EXPR
           You really probably want to be using "my" instead, because "local"
           isn't what most people think of as "local".  See "Private Variables
           via my()" in perlsub for details.

           A local modifies the listed variables to be local to the enclosing
           block, file, or eval.  If more than one value is listed, the list
           must be placed in parentheses.  See "Temporary Values via local()"
           in perlsub for details, including issues with tied arrays and

           The "delete local EXPR" construct can also be used to localize the
           deletion of array/hash elements to the current block.  See
           "Localized deletion of elements of composite types" in perlsub.

       localtime EXPR
           Converts a time as returned by the time function to a 9-element
           list with the time analyzed for the local time zone.  Typically
           used as follows:

               #  0    1    2     3     4    5     6     7     8
               ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =

           All list elements are numeric and come straight out of the C
           `struct tm'.  $sec, $min, and $hour are the seconds, minutes, and
           hours of the specified time.

           $mday is the day of the month and $mon the month in the range
           0..11, with 0 indicating January and 11 indicating December.  This
           makes it easy to get a month name from a list:

               my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );

           To get the last two digits of the year (e.g., "01" in 2001) do:

               $year = sprintf("%02d", $year % 100);

           $wday is the day of the week, with 0 indicating Sunday and 3
           indicating Wednesday.  $yday is the day of the year, in the range
           0..364 (or 0..365 in leap years.)

           $isdst is true if the specified time occurs during Daylight Saving
           Time, false otherwise.

           If EXPR is omitted, "localtime()" uses the current time (as
           returned by time(3)).

           In scalar context, "localtime()" returns the ctime(3) value:

               $now_string = localtime;  # e.g., "Thu Oct 13 04:54:34 1994"

           This scalar value is not locale-dependent but is a Perl builtin.
           For GMT instead of local time use the "gmtime" builtin. See also
           the "Time::Local" module (for converting seconds, minutes, hours,
           and such back to the integer value returned by time()), and the
           POSIX module's strftime(3) and mktime(3) functions.

           To get somewhat similar but locale-dependent date strings, set up
           your locale environment variables appropriately (please see
           perllocale) and try for example:

               use POSIX qw(strftime);
               $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
               # or for GMT formatted appropriately for your locale:
               $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;

           Note that the %a and %b, the short forms of the day of the week and
           the month of the year, may not necessarily be three characters

           See "localtime" in perlport for portability concerns.

           The Time::gmtime and Time::localtime modules provide a convenient,
           by-name access mechanism to the gmtime() and localtime() functions,

           For a comprehensive date and time representation look at the
           DateTime module on CPAN.

       lock THING
           This function places an advisory lock on a shared variable or
           referenced object contained in THING until the lock goes out of

           lock() is a "weak keyword" : this means that if you've defined a
           function by this name (before any calls to it), that function will
           be called instead.  If you are not under "use threads::shared" this

           See also "exp" for the inverse operation.

       lstat EXPR
           Does the same thing as the "stat" function (including setting the
           special "_" filehandle) but stats a symbolic link instead of the
           file the symbolic link points to.  If symbolic links are
           unimplemented on your system, a normal "stat" is done.  For much
           more detailed information, please see the documentation for "stat".

           If EXPR is omitted, stats $_.

       m// The match operator.  See "Regexp Quote-Like Operators" in perlop.

       map BLOCK LIST
       map EXPR,LIST
           Evaluates the BLOCK or EXPR for each element of LIST (locally
           setting $_ to each element) and returns the list value composed of
           the results of each such evaluation.  In scalar context, returns
           the total number of elements so generated.  Evaluates BLOCK or EXPR
           in list context, so each element of LIST may produce zero, one, or
           more elements in the returned value.

               @chars = map(chr, @numbers);

           translates a list of numbers to the corresponding characters.

               my @squares = map { $_ * $_ } @numbers;

           translates a list of numbers to their squared values.

               my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;

           shows that number of returned elements can differ from the number
           of input elements. To omit an element, return an empty list ().
           This could also be achieved by writing

               my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;

           which makes the intention more clear.

           Map always returns a list, which can be assigned to a hash such
           that the elements become key/value pairs. See perldata for more

               %hash = map { get_a_key_for($_) => $_ } @array;

           is just a funny way to write

               %hash = ();
               foreach (@array) {
                   $hash{get_a_key_for($_)} = $_;

           aliased to the list elements, $_ keeps being lexical inside the
           block; that is, it can't be seen from the outside, avoiding any
           potential side-effects.

           "{" starts both hash references and blocks, so "map { ..." could be
           either the start of map BLOCK LIST or map EXPR, LIST. Because Perl
           doesn't look ahead for the closing "}" it has to take a guess at
           which it's dealing with based on what it finds just after the "{".
           Usually it gets it right, but if it doesn't it won't realize
           something is wrong until it gets to the "}" and encounters the
           missing (or unexpected) comma. The syntax error will be reported
           close to the "}", but you'll need to change something near the "{"
           such as using a unary "+" to give Perl some help:

               %hash = map {  "\L$_" => 1  } @array  # perl guesses EXPR.  wrong
               %hash = map { +"\L$_" => 1  } @array  # perl guesses BLOCK. right
               %hash = map { ("\L$_" => 1) } @array  # this also works
               %hash = map {  lc($_) => 1  } @array  # as does this.
               %hash = map +( lc($_) => 1 ), @array  # this is EXPR and works!

               %hash = map  ( lc($_), 1 ),   @array  # evaluates to (1, @array)

           or to force an anon hash constructor use "+{":

              @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end

           to get a list of anonymous hashes each with only one entry apiece.

       mkdir FILENAME,MASK
       mkdir FILENAME
           Creates the directory specified by FILENAME, with permissions
           specified by MASK (as modified by "umask").  If it succeeds it
           returns true; otherwise it returns false and sets $! (errno).  MASK
           defaults to 0777 if omitted, and FILENAME defaults to $_ if

           In general, it is better to create directories with a permissive
           MASK and let the user modify that with their "umask" than it is to
           supply a restrictive MASK and give the user no way to be more
           permissive.  The exceptions to this rule are when the file or
           directory should be kept private (mail files, for instance).  The
           perlfunc(1) entry on "umask" discusses the choice of MASK in more

           Note that according to the POSIX 1003.1-1996 the FILENAME may have
           any number of trailing slashes.  Some operating and filesystems do
           not get this right, so Perl automatically removes all trailing
           slashes to keep everyone happy.

           To recursively create a directory structure, look at the "mkpath"
           function of the File::Path module.

       msgctl ID,CMD,ARG
       msgget KEY,FLAGS
           Calls the System V IPC function msgget(2).  Returns the message
           queue id, or "undef" on error.  See also "SysV IPC" in perlipc and
           the documentation for "IPC::SysV" and "IPC::Msg".

       msgrcv ID,VAR,SIZE,TYPE,FLAGS
           Calls the System V IPC function msgrcv to receive a message from
           message queue ID into variable VAR with a maximum message size of
           SIZE.  Note that when a message is received, the message type as a
           native long integer will be the first thing in VAR, followed by the
           actual message.  This packing may be opened with "unpack("l! a*")".
           Taints the variable.  Returns true if successful, false on error.
           See also "SysV IPC" in perlipc and the documentation for
           "IPC::SysV" and "IPC::SysV::Msg".

       msgsnd ID,MSG,FLAGS
           Calls the System V IPC function msgsnd to send the message MSG to
           the message queue ID.  MSG must begin with the native long integer
           message type, be followed by the length of the actual message, and
           then finally the message itself.  This kind of packing can be
           achieved with "pack("l! a*", $type, $message)".  Returns true if
           successful, false on error.  See also the "IPC::SysV" and
           "IPC::SysV::Msg" documentation.

       my EXPR
       my TYPE EXPR
       my EXPR : ATTRS
       my TYPE EXPR : ATTRS
           A "my" declares the listed variables to be local (lexically) to the
           enclosing block, file, or "eval".  If more than one value is
           listed, the list must be placed in parentheses.

           The exact semantics and interface of TYPE and ATTRS are still
           evolving.  TYPE is currently bound to the use of the "fields"
           pragma, and attributes are handled using the "attributes" pragma,
           or starting from Perl 5.8.0 also via the "Attribute::Handlers"
           module.  See "Private Variables via my()" in perlsub for details,
           and fields, attributes, and Attribute::Handlers.

       next LABEL
           The "next" command is like the "continue" statement in C; it starts
           the next iteration of the loop:

               LINE: while (<STDIN>) {
                   next LINE if /^#/;  # discard comments

           Note that if there were a "continue" block on the above, it would
           get executed even on discarded lines.  If LABEL is omitted, the
           command refers to the innermost enclosing loop.

           "next" cannot be used to exit a block which returns a value such as

       no MODULE
       no VERSION
           See the "use" function, of which "no" is the opposite.

       oct EXPR
       oct Interprets EXPR as an octal string and returns the corresponding
           value.  (If EXPR happens to start off with "0x", interprets it as a
           hex string.  If EXPR starts off with "0b", it is interpreted as a
           binary string.  Leading whitespace is ignored in all three cases.)
           The following will handle decimal, binary, octal, and hex in
           standard Perl notation:

               $val = oct($val) if $val =~ /^0/;

           If EXPR is omitted, uses $_.   To go the other way (produce a
           number in octal), use sprintf() or printf():

               $dec_perms = (stat("filename"))[2] & 07777;
               $oct_perm_str = sprintf "%o", $perms;

           The oct() function is commonly used when a string such as 644 needs
           to be converted into a file mode, for example.  Although Perl
           automatically converts strings into numbers as needed, this
           automatic conversion assumes base 10.

           Leading white space is ignored without warning, as too are any
           trailing non-digits, such as a decimal point ("oct" only handles
           non-negative integers, not negative integers or floating point).

       open FILEHANDLE
           Opens the file whose filename is given by EXPR, and associates it
           with FILEHANDLE.

           Simple examples to open a file for reading:

               open(my $fh, "<", "input.txt")
                   or die "cannot open < input.txt: $!";

           and for writing:

               open(my $fh, ">", "output.txt")
                   or die "cannot open > output.txt: $!";

           (The following is a comprehensive reference to open(): for a
           gentler introduction you may consider perlopentut.)

           If FILEHANDLE is an undefined scalar variable (or array or hash
           element), a new filehandle is autovivified, meaning that the
           variable is assigned a reference to a newly allocated anonymous
           filehandle.  Otherwise if FILEHANDLE is an expression, its value is
           file is opened for input.  If MODE is ">", the file is opened for
           output, with existing files first being truncated ("clobbered") and
           nonexisting files newly created.  If MODE is ">>", the file is
           opened for appending, again being created if necessary.

           You can put a "+" in front of the ">" or "<" to indicate that you
           want both read and write access to the file; thus "+<" is almost
           always preferred for read/write updates--the "+>" mode would
           clobber the file first.  You cant usually use either read-write
           mode for updating textfiles, since they have variable-length
           records.  See the -i switch in perlrun for a better approach.  The
           file is created with permissions of 0666 modified by the process's
           "umask" value.

           These various prefixes correspond to the fopen(3) modes of "r",
           "r+", "w", "w+", "a", and "a+".

           In the one- and two-argument forms of the call, the mode and
           filename should be concatenated (in that order), preferably
           separated by white space.  You can--but shouldn't--omit the mode in
           these forms when that mode is "<".  It is always safe to use the
           two-argument form of "open" if the filename argument is a known

           For three or more arguments if MODE is "|-", the filename is
           interpreted as a command to which output is to be piped, and if
           MODE is "-|", the filename is interpreted as a command that pipes
           output to us.  In the two-argument (and one-argument) form, one
           should replace dash ("-") with the command.  See "Using open() for
           IPC" in perlipc for more examples of this.  (You are not allowed to
           "open" to a command that pipes both in and out, but see IPC::Open2,
           IPC::Open3, and "Bidirectional Communication with Another Process"
           in perlipc for alternatives.)

           In the form of pipe opens taking three or more arguments, if LIST
           is specified (extra arguments after the command name) then LIST
           becomes arguments to the command invoked if the platform supports
           it.  The meaning of "open" with more than three arguments for non-
           pipe modes is not yet defined, but experimental "layers" may give
           extra LIST arguments meaning.

           In the two-argument (and one-argument) form, opening "<-" or "-"
           opens STDIN and opening ">-" opens STDOUT.

           You may (and usually should) use the three-argument form of open to
           specify I/O layers (sometimes referred to as "disciplines") to
           apply to the handle that affect how the input and output are
           processed (see open and PerlIO for more details). For example:

             open(my $fh, "<:encoding(UTF-8)", "filename")
               || die "can't open UTF-8 encoded filename: $!";

           opens the UTF8-encoded file containing Unicode characters; see
           perluniintro. Note that if layers are specified in the three-
           with a single character and encode that character in C as "\n" do
           not need "binmode".  The rest need it.

           When opening a file, it's seldom a good idea to continue if the
           request failed, so "open" is frequently used with "die".  Even if
           "die" won't do what you want (say, in a CGI script, where you want
           to format a suitable error message (but there are modules that can
           help with that problem)) always check the return value from opening
           a file.

           As a special case the three-argument form with a read/write mode
           and the third argument being "undef":

               open(my $tmp, "+>", undef) or die ...

           opens a filehandle to an anonymous temporary file.  Also using "+<"
           works for symmetry, but you really should consider writing
           something to the temporary file first.  You will need to seek() to
           do the reading.

           Since v5.8.0, Perl has built using PerlIO by default.  Unless
           you've changed this (such as building Perl with "Configure
           -Uuseperlio"), you can open filehandles directly to Perl scalars

               open($fh, ">", \$variable) || ..

           To (re)open "STDOUT" or "STDERR" as an in-memory file, close it

               close STDOUT;
               open(STDOUT, ">", \$variable)
                   or die "Can't open STDOUT: $!";

           General examples:

               $ARTICLE = 100;
               open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
               while (<ARTICLE>) {...

               open(LOG, ">>/usr/spool/news/twitlog");  # (log is reserved)
               # if the open fails, output is discarded

               open(my $dbase, "+<", "dbase.mine")      # open for update
                   or die "Can't open 'dbase.mine' for update: $!";

               open(my $dbase, "+<dbase.mine")          # ditto
                   or die "Can't open 'dbase.mine' for update: $!";

               open(ARTICLE, "-|", "caesar <$article")  # decrypt article
                   or die "Can't start caesar: $!";

               open(ARTICLE, "caesar <$article |")      # ditto
                   or die "Can't start caesar: $!";
                   process($file, "fh00");

               sub process {
                   my($filename, $input) = @_;
                   $input++;    # this is a string increment
                   unless (open($input, "<", $filename)) {
                       print STDERR "Can't open $filename: $!\n";

                   local $_;
                   while (<$input>) {    # note use of indirection
                       if (/^#include "(.*)"/) {
                           process($1, $input);
                       #...          # whatever

           See perliol for detailed info on PerlIO.

           You may also, in the Bourne shell tradition, specify an EXPR
           beginning with ">&", in which case the rest of the string is
           interpreted as the name of a filehandle (or file descriptor, if
           numeric) to be duped (as dup(2)) and opened.  You may use "&" after
           ">", ">>", "<", "+>", "+>>", and "+<".  The mode you specify should
           match the mode of the original filehandle.  (Duping a filehandle
           does not take into account any existing contents of IO buffers.) If
           you use the three-argument form, then you can pass either a number,
           the name of a filehandle, or the normal "reference to a glob".

           Here is a script that saves, redirects, and restores "STDOUT" and
           "STDERR" using various methods:

               open(my $oldout, ">&STDOUT")     or die "Can't dup STDOUT: $!";
               open(OLDERR,     ">&", \*STDERR) or die "Can't dup STDERR: $!";

               open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
               open(STDERR, ">&STDOUT")     or die "Can't dup STDOUT: $!";

               select STDERR; $| = 1;  # make unbuffered
               select STDOUT; $| = 1;  # make unbuffered

               print STDOUT "stdout 1\n";  # this works for
               print STDERR "stderr 1\n";  # subprocesses too

               open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
               open(STDERR, ">&OLDERR")    or die "Can't dup OLDERR: $!";

               print STDOUT "stdout 2\n";
               print STDERR "stderr 2\n";


               # open for append, using the fileno of OLDFH
               open(FH, ">&gt;&=", OLDFH)


               open(FH, ">&gt;&=OLDFH")

           Being parsimonious on filehandles is also useful (besides being
           parsimonious) for example when something is dependent on file
           descriptors, like for example locking using flock().  If you do
           just "open(A, ">&gt;&B")", the filehandle A will not have the same
           file descriptor as B, and therefore flock(A) will not flock(B) nor
           vice versa.  But with "open(A, ">&gt;&=B")", the filehandles will
           share the same underlying system file descriptor.

           Note that under Perls older than 5.8.0, Perl uses the standard C
           library's' fdopen() to implement the "=" functionality.  On many
           Unix systems, fdopen() fails when file descriptors exceed a certain
           value, typically 255.  For Perls 5.8.0 and later, PerlIO is (most
           often) the default.

           You can see whether your Perl was built with PerlIO by running
           "perl -V" and looking for the "useperlio=" line.  If "useperlio" is
           "define", you have PerlIO; otherwise you don't.

           If you open a pipe on the command "-" (that is, specify either "|-"
           or "-|" with the one- or two-argument forms of "open"), an implicit
           "fork" is done, so "open" returns twice: in the parent process it
           returns the pid of the child process, and in the child process it
           returns (a defined) 0.  Use "defined($pid)" or "//" to determine
           whether the open was successful.

           For example, use either

               $child_pid = open(FROM_KID, "|-")   // die "can't fork: $!";

               $child_pid = open(TO_KID,   "|-")   // die "can't fork: $!";

           followed by

               if ($child_pid) {
                   # am the parent:
                   # either write TO_KID or else read FROM_KID
                   wait $child_pid;
               } else {
                   # am the child; use STDIN/STDOUT normally
               open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
               open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
               open(FOO, "|-", "tr", '[a-z]', '[A-Z]');

               open(FOO, "cat -n '$file'|");
               open(FOO, "-|", "cat -n '$file'");
               open(FOO, "-|") || exec "cat", "-n", $file;
               open(FOO, "-|", "cat", "-n", $file);

           The last two examples in each block show the pipe as "list form",
           which is not yet supported on all platforms.  A good rule of thumb
           is that if your platform has a real "fork()" (in other words, if
           your platform is Unix, including Linux and MacOS X), you can use
           the list form.  You would want to use the list form of the pipe so
           you can pass literal arguments to the command without risk of the
           shell interpreting any shell metacharacters in them.  However, this
           also bars you from opening pipes to commands that intentionally
           contain shell metacharacters, such as:

               open(FOO, "|cat -n | expand -4 | lpr")
                   // die "Can't open pipeline to lpr: $!";

           See "Safe Pipe Opens" in perlipc for more examples of this.

           Beginning with v5.6.0, Perl will attempt to flush all files opened
           for output before any operation that may do a fork, but this may
           not be supported on some platforms (see perlport).  To be safe, you
           may need to set $| ($AUTOFLUSH in English) or call the
           "autoflush()" method of "IO::Handle" on any open handles.

           On systems that support a close-on-exec flag on files, the flag
           will be set for the newly opened file descriptor as determined by
           the value of $^F.  See "$^F" in perlvar.

           Closing any piped filehandle causes the parent process to wait for
           the child to finish, then returns the status value in $? and

           The filename passed to the one- and two-argument forms of open()
           will have leading and trailing whitespace deleted and normal
           redirection characters honored.  This property, known as "magic
           open", can often be used to good effect.  A user could specify a
           filename of "rsh cat file |", or you could change certain filenames
           as needed:

               $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
               open(FH, $filename) or die "Can't open $filename: $!";

           Use the three-argument form to open a file with arbitrary weird
           characters in it,

               open(FOO, "<", $file)
                   || die "can't open < $file: $!";

           will allow the user to specify an argument of the form "rsh cat
           file |", but will not work on a filename that happens to have a
           trailing space, while

               open(IN, "<", $ARGV[0])
                   || die "can't open < $ARGV[0]: $!";

           will have exactly the opposite restrictions.

           If you want a "real" C "open" (see open(2) on your system), then
           you should use the "sysopen" function, which involves no such magic
           (but may use subtly different filemodes than Perl open(), which is
           mapped to C fopen()).  This is another way to protect your
           filenames from interpretation.  For example:

               use IO::Handle;
               sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
                   or die "sysopen $path: $!";
               $oldfh = select(HANDLE); $| = 1; select($oldfh);
               print HANDLE "stuff $$\n";
               seek(HANDLE, 0, 0);
               print "File contains: ", <HANDLE>;

           Using the constructor from the "IO::Handle" package (or one of its
           subclasses, such as "IO::File" or "IO::Socket"), you can generate
           anonymous filehandles that have the scope of the variables used to
           hold them, then automatically (but silently) close once their
           reference counts become zero, typically at scope exit:

               use IO::File;
               sub read_myfile_munged {
                   my $ALL = shift;
                   # or just leave it undef to autoviv
                   my $handle = IO::File->new;
                   open($handle, "<", "myfile") or die "myfile: $!";
                   $first = <$handle>
                       or return ();     # Automatically closed here.
                   mung($first) or die "mung failed";  # Or here.
                   return (first, <$handle>) if $ALL;  # Or here.
                   return $first;                      # Or here.

           WARNING: The previous example has a bug because the automatic close
           that happens when the refcount on "handle" does not properly detect
           and report failures.  Always close the handle yourself and inspect
           the return value.

                   || warn "close failed: $!";

           See "seek" for some details about mixing reading and writing.

       ord EXPR
       ord Returns the numeric (the native 8-bit encoding, like ASCII or
           EBCDIC, or Unicode) value of the first character of EXPR.  If EXPR
           is an empty string, returns 0.  If EXPR is omitted, uses $_.  (Note
           character, not byte.)

           For the reverse, see "chr".  See perlunicode for more about

       our EXPR
       our TYPE EXPR
       our EXPR : ATTRS
       our TYPE EXPR : ATTRS
           "our" associates a simple name with a package variable in the
           current package for use within the current scope.  When "use strict
           'vars'" is in effect, "our" lets you use declared global variables
           without qualifying them with package names, within the lexical
           scope of the "our" declaration.  In this way "our" differs from
           "use vars", which is package-scoped.

           Unlike "my" or "state", which allocates storage for a variable and
           associates a simple name with that storage for use within the
           current scope, "our" associates a simple name with a package (read:
           global) variable in the current package, for use within the current
           lexical scope.  In other words, "our" has the same scoping rules as
           "my" or "state", but does not necessarily create a variable.

           If more than one value is listed, the list must be placed in

               our $foo;
               our($bar, $baz);

           An "our" declaration declares a global variable that will be
           visible across its entire lexical scope, even across package
           boundaries.  The package in which the variable is entered is
           determined at the point of the declaration, not at the point of
           use.  This means the following behavior holds:

               package Foo;
               our $bar;      # declares $Foo::bar for rest of lexical scope
               $bar = 20;

               package Bar;
               print $bar;    # prints 20, as it refers to $Foo::bar

           Multiple "our" declarations with the same name in the same lexical
           scope are allowed if they are in different packages.  If they
           happen to be in the same package, Perl will emit warnings if you
           have asked for them, just like multiple "my" declarations.  Unlike
           a second "my" declaration, which will bind the name to a fresh
           variable, a second "our" declaration in the same package, in the
           same scope, is merely redundant.

           An "our" declaration may also have a list of attributes associated
           with it.

           The exact semantics and interface of TYPE and ATTRS are still
           evolving.  TYPE is currently bound to the use of "fields" pragma,
           and attributes are handled using the "attributes" pragma, or
           starting from Perl 5.8.0 also via the "Attribute::Handlers" module.
           See "Private Variables via my()" in perlsub for details, and
           fields, attributes, and Attribute::Handlers.

       pack TEMPLATE,LIST
           Takes a LIST of values and converts it into a string using the
           rules given by the TEMPLATE.  The resulting string is the
           concatenation of the converted values.  Typically, each converted
           value looks like its machine-level representation.  For example, on
           32-bit machines an integer may be represented by a sequence of 4
           bytes, which  will in Perl be presented as a string that's 4
           characters long.

           See perlpacktut for an introduction to this function.

           The TEMPLATE is a sequence of characters that give the order and
           type of values, as follows:

               a  A string with arbitrary binary data, will be null padded.
               A  A text (ASCII) string, will be space padded.
               Z  A null-terminated (ASCIZ) string, will be null padded.

               b  A bit string (ascending bit order inside each byte, like vec()).
               B  A bit string (descending bit order inside each byte).
               h  A hex string (low nybble first).
               H  A hex string (high nybble first).

               c  A signed char (8-bit) value.
               C  An unsigned char (octet) value.
               W  An unsigned char value (can be greater than 255).

               s  A signed short (16-bit) value.
               S  An unsigned short value.

               l  A signed long (32-bit) value.
               L  An unsigned long value.

               q  A signed quad (64-bit) value.
               Q  An unsigned quad value.
                 (Quads are available only if your system supports 64-bit
                  integer values _and_ if Perl has been compiled to support those.
                      Raises an exception otherwise.)

               i  A signed integer value.
               I  A unsigned integer value.
                 (This 'integer' is _at_least_ 32 bits wide.  Its exact
                      size depends on what a local C compiler calls 'int'.)

               D  A float of long-double precision in native format.
                 (Long doubles are available only if your system supports long
                  double values _and_ if Perl has been compiled to support those.
                      Raises an exception otherwise.)

               p  A pointer to a null-terminated string.
               P  A pointer to a structure (fixed-length string).

               u  A uuencoded string.
               U  A Unicode character number.  Encodes to a character in character mode
                   and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode.

               w  A BER compressed integer (not an ASN.1 BER, see perlpacktut for
                  details).  Its bytes represent an unsigned integer in base 128,
                  most significant digit first, with as few digits as possible.  Bit
                  eight (the high bit) is set on each byte except the last.

               x  A null byte (a.k.a ASCII NUL, "\000", chr(0))
               X  Back up a byte.
               @  Null-fill or truncate to absolute position, counted from the
                  start of the innermost ()-group.
               .  Null-fill or truncate to absolute position specified by the value.
               (  Start of a ()-group.

           One or more modifiers below may optionally follow certain letters
           in the TEMPLATE (the second column lists letters for which the
           modifier is valid):

               !   sSlLiI     Forces native (short, long, int) sizes instead
                              of fixed (16-/32-bit) sizes.

                   xX         Make x and X act as alignment commands.

                   nNvV       Treat integers as signed instead of unsigned.

                   @.         Specify position as byte offset in the internal
                              representation of the packed string. Efficient but

               >   sSiIlLqQ   Force big-endian byte-order on the type.
                   jJfFdDpP   (The "big end" touches the construct.)

               <   sSiIlLqQ   Force little-endian byte-order on the type.
                   jJfFdDpP   (The "little end" touches the construct.)

           The ">" and "<" modifiers can also be used on "()" groups to force
           a particular byte-order on all components in that group, including
           all its subgroups.

           The following rules apply:

           o   Each letter may optionally be followed by a number indicating
               the repeat count.  A numeric repeat count may optionally be
               enclosed in brackets, as in "pack("C[80]", @arr)".  The repeat

               One can replace a numeric repeat count with a template letter
               enclosed in brackets to use the packed byte length of the
               bracketed template for the repeat count.

               For example, the template "x[L]" skips as many bytes as in a
               packed long, and the template "$t X[$t] $t" unpacks twice
               whatever $t (when variable-expanded) unpacks.  If the template
               in brackets contains alignment commands (such as "x![d]"), its
               packed length is calculated as if the start of the template had
               the maximal possible alignment.

               When used with "Z", a "*" as the repeat count is guaranteed to
               add a trailing null byte, so the resulting string is always one
               byte longer than the byte length of the item itself.

               When used with "@", the repeat count represents an offset from
               the start of the innermost "()" group.

               When used with ".", the repeat count determines the starting
               position to calculate the value offset as follows:

               o   If the repeat count is 0, it's relative to the current

               o   If the repeat count is "*", the offset is relative to the
                   start of the packed string.

               o   And if it's an integer n, the offset is relative to the
                   start of the nth innermost "( )" group, or to the start of
                   the string if n is bigger then the group level.

               The repeat count for "u" is interpreted as the maximal number
               of bytes to encode per line of output, with 0, 1 and 2 replaced
               by 45. The repeat count should not be more than 65.

           o   The "a", "A", and "Z" types gobble just one value, but pack it
               as a string of length count, padding with nulls or spaces as
               needed.  When unpacking, "A" strips trailing whitespace and
               nulls, "Z" strips everything after the first null, and "a"
               returns data with no stripping at all.

               If the value to pack is too long, the result is truncated.  If
               it's too long and an explicit count is provided, "Z" packs only
               "$count-1" bytes, followed by a null byte.  Thus "Z" always
               packs a trailing null, except when the count is 0.

           o   Likewise, the "b" and "B" formats pack a string that's that
               many bits long.  Each such format generates 1 bit of the
               result.  These are typically followed by a repeat count like
               "B8" or "B64".

               Each result bit is based on the least-significant bit of the
               null characters at the end.  Similarly during unpacking,
               "extra" bits are ignored.

               If the input string is longer than needed, remaining characters
               are ignored.

               A "*" for the repeat count uses all characters of the input
               field.  On unpacking, bits are converted to a string of 0s and

           o   The "h" and "H" formats pack a string that many nybbles (4-bit
               groups, representable as hexadecimal digits, "0".."9" "a".."f")

               For each such format, pack() generates 4 bits of result.  With
               non-alphabetical characters, the result is based on the 4
               least-significant bits of the input character, i.e., on
               "ord($char)%16".  In particular, characters "0" and "1"
               generate nybbles 0 and 1, as do bytes "\000" and "\001".  For
               characters "a".."f" and "A".."F", the result is compatible with
               the usual hexadecimal digits, so that "a" and "A" both generate
               the nybble "0xA==10".  Use only these specific hex characters
               with this format.

               Starting from the beginning of the template to pack(), each
               pair of characters is converted to 1 character of output.  With
               format "h", the first character of the pair determines the
               least-significant nybble of the output character; with format
               "H", it determines the most-significant nybble.

               If the length of the input string is not even, it behaves as if
               padded by a null character at the end.  Similarly, "extra"
               nybbles are ignored during unpacking.

               If the input string is longer than needed, extra characters are

               A "*" for the repeat count uses all characters of the input
               field.  For unpack(), nybbles are converted to a string of
               hexadecimal digits.

           o   The "p" format packs a pointer to a null-terminated string.
               You are responsible for ensuring that the string is not a
               temporary value, as that could potentially get deallocated
               before you got around to using the packed result.  The "P"
               format packs a pointer to a structure of the size indicated by
               the length.  A null pointer is created if the corresponding
               value for "p" or "P" is "undef"; similarly with unpack(), where
               a null pointer unpacks into "undef".

               If your system has a strange pointer size--meaning a pointer is
               neither as big as an int nor as big as a long--it may not be
               possible to pack or unpack pointers in big- or little-endian
               byte order.  Attempting to do so raises an exception.

               For "pack", sequence-item may have a repeat count, in which
               case the minimum of that and the number of available items is
               used as the argument for length-item. If it has no repeat count
               or uses a '*', the number of available items is used.

               For "unpack", an internal stack of integer arguments unpacked
               so far is used. You write "/"sequence-item and the repeat count
               is obtained by popping off the last element from the stack. The
               sequence-item must not have a repeat count.

               If sequence-item refers to a string type ("A", "a", or "Z"),
               the length-item is the string length, not the number of
               strings.  With an explicit repeat count for pack, the packed
               string is adjusted to that length.  For example:

                   unpack("W/a", "\004Gurusamy")           gives ("Guru")
                   unpack("a3/A A*", "007 Bond  J ")       gives (" Bond", "J")
                   unpack("a3 x2 /A A*", "007: Bond, J.")  gives ("Bond, J", ".")

                   pack("n/a* w/a","hello,","world")       gives "\000\006hello,\005world"
                   pack("a/W2", ord("a") .. ord("z"))      gives "2ab"

               The length-item is not returned explicitly from "unpack".

               Supplying a count to the length-item format letter is only
               useful with "A", "a", or "Z".  Packing with a length-item of
               "a" or "Z" may introduce "\000" characters, which Perl does not
               regard as legal in numeric strings.

           o   The integer types "s", "S", "l", and "L" may be followed by a
               "!" modifier to specify native shorts or longs.  As shown in
               the example above, a bare "l" means exactly 32 bits, although
               the native "long" as seen by the local C compiler may be
               larger.  This is mainly an issue on 64-bit platforms.  You can
               see whether using "!" makes any difference this way:

                   printf "format s is %d, s! is %d\n",
                       length pack("s"), length pack("s!");

                   printf "format l is %d, l! is %d\n",
                       length pack("l"), length pack("l!");

               "i!" and "I!" are also allowed, but only for completeness'
               sake: they are identical to "i" and "I".

               The actual sizes (in bytes) of native shorts, ints, longs, and
               long longs on the platform where Perl was built are also
               available from the command line:

                   $ perl -V:{short,int,long{,long}}size

           o   The integer formats "s", "S", "i", "I", "l", "L", "j", and "J"
               are inherently non-portable between processors and operating
               systems because they obey native byteorder and endianness.  For
               example, a 4-byte integer 0x12345678 (305419896 decimal) would
               be ordered natively (arranged in and handled by the CPU
               registers) into bytes as

                   0x12 0x34 0x56 0x78  # big-endian
                   0x78 0x56 0x34 0x12  # little-endian

               Basically, Intel and VAX CPUs are little-endian, while
               everybody else, including Motorola m68k/88k, PPC, Sparc, HP PA,
               Power, and Cray, are big-endian.  Alpha and MIPS can be either:
               Digital/Compaq uses (well, used) them in little-endian mode,
               but SGI/Cray uses them in big-endian mode.

               The names big-endian and little-endian are comic references to
               the egg-eating habits of the little-endian Lilliputians and the
               big-endian Blefuscudians from the classic Jonathan Swift
               satire, Gulliver's Travels.  This entered computer lingo via
               the paper "On Holy Wars and a Plea for Peace" by Danny Cohen,
               USC/ISI IEN 137, April 1, 1980.

               Some systems may have even weirder byte orders such as

                  0x56 0x78 0x12 0x34
                  0x34 0x12 0x78 0x56

               You can determine your system endianness with this incantation:

                  printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);

               The byteorder on the platform where Perl was built is also
               available via Config:

                   use Config;
                   print "$Config{byteorder}\n";

               or from the command line:

                   $ perl -V:byteorder

               Byteorders "1234" and "12345678" are little-endian; "4321" and
               "87654321" are big-endian.

               For portably packed integers, either use the formats "n", "N",
               "v", and "V" or else use the ">" and "<" modifiers described
               immediately below.  See also perlport.

           o   Starting with Perl 5.9.2, integer and floating-point formats,
               along with the "p" and "P" formats and "()" groups, may all be
               followed by the ">" or "<" endianness modifiers to respectively
               enforce big- or little-endian byte-order.  These modifiers are
                   formats on big- or little-endian machines.  Otherwise,
                   attempting to use them raises an exception.

               o   Forcing big- or little-endian byte-order on floating-point
                   values for data exchange can work only if all platforms use
                   the same binary representation such as IEEE floating-point.
                   Even if all platforms are using IEEE, there may still be
                   subtle differences.  Being able to use ">" or "<" on
                   floating-point values can be useful, but also dangerous if
                   you don't know exactly what you're doing.  It is not a
                   general way to portably store floating-point values.

               o   When using ">" or "<" on a "()" group, this affects all
                   types inside the group that accept byte-order modifiers,
                   including all subgroups.  It is silently ignored for all
                   other types.  You are not allowed to override the byte-
                   order within a group that already has a byte-order modifier

           o   Real numbers (floats and doubles) are in native machine format
               only.  Due to the multiplicity of floating-point formats and
               the lack of a standard "network" representation for them, no
               facility for interchange has been made.  This means that packed
               floating-point data written on one machine may not be readable
               on another, even if both use IEEE floating-point arithmetic
               (because the endianness of the memory representation is not
               part of the IEEE spec).  See also perlport.

               If you know exactly what you're doing, you can use the ">" or
               "<" modifiers to force big- or little-endian byte-order on
               floating-point values.

               Because Perl uses doubles (or long doubles, if configured)
               internally for all numeric calculation, converting from double
               into float and thence to double again loses precision, so
               "unpack("f", pack("f", $foo)") will not in general equal $foo.

           o   Pack and unpack can operate in two modes: character mode ("C0"
               mode) where the packed string is processed per character, and
               UTF-8 mode ("U0" mode) where the packed string is processed in
               its UTF-8-encoded Unicode form on a byte-by-byte basis.
               Character mode is the default unless the format string starts
               with "U". You can always switch mode mid-format with an
               explicit "C0" or "U0" in the format.  This mode remains in
               effect until the next mode change, or until the end of the "()"
               group it (directly) applies to.

               Using "C0" to get Unicode characters while using "U0" to get
               non-Unicode bytes is not necessarily obvious.   Probably only
               the first of these is what you want:

                   $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
                     perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'

           o   You must yourself do any alignment or padding by inserting, for
               example, enough "x"es while packing.  There is no way for
               pack() and unpack() to know where characters are going to or
               coming from, so they handle their output and input as flat
               sequences of characters.

           o   A "()" group is a sub-TEMPLATE enclosed in parentheses.  A
               group may take a repeat count either as postfix, or for
               unpack(), also via the "/" template character.  Within each
               repetition of a group, positioning with "@" starts over at 0.
               Therefore, the result of

                   pack("@1A((@2A)@3A)", qw[X Y Z])

               is the string "\0X\0\0YZ".

           o   "x" and "X" accept the "!" modifier to act as alignment
               commands: they jump forward or back to the closest position
               aligned at a multiple of "count" characters. For example, to
               pack() or unpack() a C structure like

                   struct {
                       char   c;    /* one signed, 8-bit character */
                       double d;
                       char   cc[2];

               one may need to use the template "c x![d] d c[2]".  This
               assumes that doubles must be aligned to the size of double.

               For alignment commands, a "count" of 0 is equivalent to a
               "count" of 1; both are no-ops.

           o   "n", "N", "v" and "V" accept the "!" modifier to represent
               signed 16-/32-bit integers in big-/little-endian order.  This
               is portable only when all platforms sharing packed data use the
               same binary representation for signed integers; for example,
               when all platforms use two's-complement representation.

           o   Comments can be embedded in a TEMPLATE using "#" through the
               end of line.  White space can separate pack codes from each
               other, but modifiers and repeat counts must follow immediately.
               Breaking complex templates into individual line-by-line
               components, suitably annotated, can do as much to improve
               legibility and maintainability of pack/unpack formats as "/x"
               can for complicated pattern matches.

           o   If TEMPLATE requires more arguments than pack() is given,
               pack() assumes additional "" arguments.  If TEMPLATE requires
               fewer arguments than given, extra arguments are ignored.


               # This is the UTF-8 encoding of the string in the previous example

               $foo = pack("ccxxcc",65,66,67,68);
               # foo eq "AB\0\0CD"

               # NOTE: The examples above featuring "W" and "c" are true
               # only on ASCII and ASCII-derived systems such as ISO Latin 1
               # and UTF-8.  On EBCDIC systems, the first example would be
               #      $foo = pack("WWWW",193,194,195,196);

               $foo = pack("s2",1,2);
               # "\001\000\002\000" on little-endian
               # "\000\001\000\002" on big-endian

               $foo = pack("a4","abcd","x","y","z");
               # "abcd"

               $foo = pack("aaaa","abcd","x","y","z");
               # "axyz"

               $foo = pack("a14","abcdefg");
               # "abcdefg\0\0\0\0\0\0\0"

               $foo = pack("i9pl", gmtime);
               # a real struct tm (on my system anyway)

               $utmp_template = "Z8 Z8 Z16 L";
               $utmp = pack($utmp_template, @utmp1);
               # a struct utmp (BSDish)

               @utmp2 = unpack($utmp_template, $utmp);
               # "@utmp1" eq "@utmp2"

               sub bintodec {
                   unpack("N", pack("B32", substr("0" x 32 . shift, -32)));

               $foo = pack('sx2l', 12, 34);
               # short 12, two zero bytes padding, long 34
               $bar = pack('s@4l', 12, 34);
               # short 12, zero fill to position 4, long 34
               # $foo eq $bar
               $baz = pack('s.l', 12, 4, 34);
               # short 12, zero fill to position 4, long 34

               $foo = pack('nN', 42, 4711);
               # pack big-endian 16- and 32-bit unsigned integers
               $foo = pack('S>L>', 42, 4711);
               # exactly the same
               $foo = pack('s<l<', -42, 4711);
               # pack little-endian 16- and 32-bit signed integers
               $foo = pack('(sl)<', -42, 4711);
               # exactly the same

           the "my", "state", and "our" operators.  All unqualified dynamic
           identifiers in this scope will be in the given namespace, except
           where overridden by another "package" declaration or when they're
           one of the special identifiers that qualify into "main::", like
           "STDOUT", "ARGV", "ENV", and the punctuation variables.

           A package statement affects dynamic variables only, including those
           you've used "local" on, but not lexical variables, which are
           created with "my", "state", or "our".  Typically it would be the
           first declaration in a file included by "require" or "use".  You
           can switch into a package in more than one place, since this only
           determines which default symbol table the compiler uses for the
           rest of that block.  You can refer to identifiers in other packages
           than the current one by prefixing the identifier with the package
           name and a double colon, as in $SomePack::var or
           "ThatPack::INPUT_HANDLE".  If package name is omitted, the "main"
           package as assumed.  That is, $::sail is equivalent to $main::sail
           (as well as to "$main'sail", still seen in ancient code, mostly
           from Perl 4).

           If VERSION is provided, "package" sets the $VERSION variable in the
           given namespace to a version object with the VERSION provided.
           VERSION must be a "strict" style version number as defined by the
           version module: a positive decimal number (integer or decimal-
           fraction) without exponentiation or else a dotted-decimal v-string
           with a leading 'v' character and at least three components.  You
           should set $VERSION only once per package.

           See "Packages" in perlmod for more information about packages,
           modules, and classes.  See perlsub for other scoping issues.

           Opens a pair of connected pipes like the corresponding system call.
           Note that if you set up a loop of piped processes, deadlock can
           occur unless you are very careful.  In addition, note that Perl's
           pipes use IO buffering, so you may need to set $| to flush your
           WRITEHANDLE after each command, depending on the application.

           See IPC::Open2, IPC::Open3, and "Bidirectional Communication with
           Another Process" in perlipc for examples of such things.

           On systems that support a close-on-exec flag on files, that flag is
           set on all newly opened file descriptors whose "fileno"s are higher
           than the current value of $^F (by default 2 for "STDERR").  See
           "$^F" in perlvar.

       pop ARRAY
       pop EXPR
       pop Pops and returns the last value of the array, shortening the array
           by one element.

           Returns the undefined value if the array is empty, although this
           may also happen at other times.  If ARRAY is omitted, pops the
           @ARGV array in the main program, but the @_ array in subroutines,
           that the search position is reset (usually due to match failure,
           but can also be because no match has yet been run on the scalar).

           "pos" directly accesses the location used by the regexp engine to
           store the offset, so assigning to "pos" will change that offset,
           and so will also influence the "\G" zero-width assertion in regular
           expressions. Both of these effects take place for the next match,
           so you can't affect the position with "pos" during the current
           match, such as in "(?{pos() = 5})" or "s//pos() = 5/e".

           Setting "pos" also resets the matched with zero-length flag,
           described under "Repeated Patterns Matching a Zero-length
           Substring" in perlre.

           Because a failed "m//gc" match doesn't reset the offset, the return
           from "pos" won't change either in this case.  See perlre and

       print FILEHANDLE LIST
       print FILEHANDLE
       print LIST
           Prints a string or a list of strings.  Returns true if successful.
           FILEHANDLE may be a scalar variable containing the name of or a
           reference to the filehandle, thus introducing one level of
           indirection.  (NOTE: If FILEHANDLE is a variable and the next token
           is a term, it may be misinterpreted as an operator unless you
           interpose a "+" or put parentheses around the arguments.) If
           FILEHANDLE is omitted, prints to the last selected (see "select")
           output handle.  If LIST is omitted, prints $_ to the currently
           selected output handle.  To use FILEHANDLE alone to print the
           content of $_ to it, you must use a real filehandle like "FH", not
           an indirect one like $fh.  To set the default output handle to
           something other than STDOUT, use the select operation.

           The current value of $, (if any) is printed between each LIST item.
           The current value of "$\" (if any) is printed after the entire LIST
           has been printed.  Because print takes a LIST, anything in the LIST
           is evaluated in list context, including any subroutines whose
           return lists you pass to "print".  Be careful not to follow the
           print keyword with a left parenthesis unless you want the
           corresponding right parenthesis to terminate the arguments to the
           print; put parentheses around all arguments (or interpose a "+",
           but that doesn't look as good).

           If you're storing handles in an array or hash, or in general
           whenever you're using any expression more complex than a bareword
           handle or a plain, unsubscripted scalar variable to retrieve it,
           you will have to use a block returning the filehandle value
           instead, in which case the LIST may not be omitted:

               print { $files[$i] } "stuff\n";
               print { $OK ? STDOUT : STDERR } "stuff\n";

           must use a real filehandle like "FH", not an indirect one like $fh.
           If "use locale" is in effect and POSIX::setlocale() has been
           called, the character used for the decimal separator in formatted
           floating-point numbers is affected by the LC_NUMERIC locale
           setting.  See perllocale and POSIX.

           Don't fall into the trap of using a "printf" when a simple "print"
           would do.  The "print" is more efficient and less error prone.

       prototype FUNCTION
           Returns the prototype of a function as a string (or "undef" if the
           function has no prototype).  FUNCTION is a reference to, or the
           name of, the function whose prototype you want to retrieve.

           If FUNCTION is a string starting with "CORE::", the rest is taken
           as a name for a Perl builtin.  If the builtin is not overridable
           (such as "qw//") or if its arguments cannot be adequately expressed
           by a prototype (such as "system"), prototype() returns "undef",
           because the builtin does not really behave like a Perl function.
           Otherwise, the string describing the equivalent prototype is

       push ARRAY,LIST
       push EXPR,LIST
           Treats ARRAY as a stack by appending the values of LIST to the end
           of ARRAY.  The length of ARRAY increases by the length of LIST.
           Has the same effect as

               for $value (LIST) {
                   $ARRAY[++$#ARRAY] = $value;

           but is more efficient.  Returns the number of elements in the array
           following the completed "push".

           Starting with Perl 5.14, "push" can take a scalar EXPR, which must
           hold a reference to an unblessed array.  The argument will be
           dereferenced automatically.  This aspect of "push" is considered
           highly experimental.  The exact behaviour may change in a future
           version of Perl.

           Generalized quotes.  See "Quote-Like Operators" in perlop.

           Regexp-like quote.  See "Regexp Quote-Like Operators" in perlop.

       quotemeta EXPR
           Returns the value of EXPR with all non-"word" characters
           backslashed.  (That is, all characters not matching
               $sentence =~ s{$substring}{big bad wolf};

           Will cause $sentence to become 'The big bad wolf jumped over...'.

           On the other hand:

               my $sentence = 'The quick brown fox jumped over the lazy dog';
               my $substring = 'quick.*?fox';
               $sentence =~ s{\Q$substring\E}{big bad wolf};


               my $sentence = 'The quick brown fox jumped over the lazy dog';
               my $substring = 'quick.*?fox';
               my $quoted_substring = quotemeta($substring);
               $sentence =~ s{$quoted_substring}{big bad wolf};

           Will both leave the sentence as is. Normally, when accepting
           literal string input from the user, quotemeta() or "\Q" must be

           In Perl 5.14, all characters whose code points are above 127 are
           not quoted in UTF8-encoded strings, but all are quoted in UTF-8
           strings.  It is planned to change this behavior in 5.16, but the
           exact rules haven't been determined yet.

       rand EXPR
           Returns a random fractional number greater than or equal to 0 and
           less than the value of EXPR.  (EXPR should be positive.)  If EXPR
           is omitted, the value 1 is used.  Currently EXPR with the value 0
           is also special-cased as 1 (this was undocumented before Perl 5.8.0
           and is subject to change in future versions of Perl).
           Automatically calls "srand" unless "srand" has already been called.
           See also "srand".

           Apply "int()" to the value returned by "rand()" if you want random
           integers instead of random fractional numbers.  For example,


           returns a random integer between 0 and 9, inclusive.

           (Note: If your rand function consistently returns numbers that are
           too large or too small, then your version of Perl was probably
           compiled with the wrong number of RANDBITS.)

           "rand()" is not cryptographically secure.  You should not rely on
           it in security-sensitive situations.  As of this writing, a number
           of third-party CPAN modules offer random number generators intended
           by their authors to be cryptographically secure, including:
           Math::Random::Secure, Math::Random::MT::Perl, and

           of the string.  A positive OFFSET greater than the length of SCALAR
           results in the string being padded to the required size with "\0"
           bytes before the result of the read is appended.

           The call is implemented in terms of either Perl's or your system's
           native fread(3) library function.  To get a true read(2) system
           call, see "sysread".

           Note the characters: depending on the status of the filehandle,
           either (8-bit) bytes or characters are read.  By default, all
           filehandles operate on bytes, but for example if the filehandle has
           been opened with the ":utf8" I/O layer (see "open", and the "open"
           pragma, open), the I/O will operate on UTF8-encoded Unicode
           characters, not bytes.  Similarly for the ":encoding" pragma: in
           that case pretty much any characters can be read.

       readdir DIRHANDLE
           Returns the next directory entry for a directory opened by
           "opendir".  If used in list context, returns all the rest of the
           entries in the directory.  If there are no more entries, returns
           the undefined value in scalar context and the empty list in list

           If you're planning to filetest the return values out of a
           "readdir", you'd better prepend the directory in question.
           Otherwise, because we didn't "chdir" there, it would have been
           testing the wrong file.

               opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
               @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
               closedir $dh;

           As of Perl 5.11.2 you can use a bare "readdir" in a "while" loop,
           which will set $_ on every iteration.

               opendir(my $dh, $some_dir) || die;
               while(readdir $dh) {
                   print "$some_dir/$_\n";
               closedir $dh;

       readline EXPR
           Reads from the filehandle whose typeglob is contained in EXPR (or
           from *ARGV if EXPR is not provided).  In scalar context, each call
           reads and returns the next line until end-of-file is reached,
           whereupon the subsequent call returns "undef".  In list context,
           reads until end-of-file is reached and returns a list of lines.
           Note that the notion of "line" used here is whatever you may have
           defined with $/ or $INPUT_RECORD_SEPARATOR).  See "$/" in perlvar.

           When $/ is set to "undef", when "readline" is in scalar context
           (i.e., file slurp mode), and when an empty file is read, it returns
           '' the first time, followed by "undef" subsequently.
           "readline" and dies if the result is not defined.

               while ( ! eof($fh) ) {
                   defined( $_ = <$fh> ) or die "readline failed: $!";

           Note that you have can't handle "readline" errors that way with the
           "ARGV" filehandle. In that case, you have to open each element of
           @ARGV yourself since "eof" handles "ARGV" differently.

               foreach my $arg (@ARGV) {
                   open(my $fh, $arg) or warn "Can't open $arg: $!";

                   while ( ! eof($fh) ) {
                       defined( $_ = <$fh> )
                           or die "readline failed for $arg: $!";

       readlink EXPR
           Returns the value of a symbolic link, if symbolic links are
           implemented.  If not, raises an exception.  If there is a system
           error, returns the undefined value and sets $! (errno).  If EXPR is
           omitted, uses $_.

       readpipe EXPR
           EXPR is executed as a system command.  The collected standard
           output of the command is returned.  In scalar context, it comes
           back as a single (potentially multi-line) string.  In list context,
           returns a list of lines (however you've defined lines with $/ or
           $INPUT_RECORD_SEPARATOR).  This is the internal function
           implementing the "qx/EXPR/" operator, but you can use it directly.
           The "qx/EXPR/" operator is discussed in more detail in "I/O
           Operators" in perlop.  If EXPR is omitted, uses $_.

           Receives a message on a socket.  Attempts to receive LENGTH
           characters of data into variable SCALAR from the specified SOCKET
           filehandle.  SCALAR will be grown or shrunk to the length actually
           read.  Takes the same flags as the system call of the same name.
           Returns the address of the sender if SOCKET's protocol supports
           this; returns an empty string otherwise.  If there's an error,
           returns the undefined value.  This call is actually implemented in
           terms of recvfrom(2) system call.  See "UDP: Message Passing" in
           perlipc for examples.

           Note the characters: depending on the status of the socket, either
           (8-bit) bytes or characters are received.  By default all sockets
           operate on bytes, but for example if the socket has been changed
           using binmode() to operate with the ":encoding(utf8)" I/O layer
               # a simpleminded Pascal comment stripper
               # (warning: assumes no { or } in strings)
               LINE: while (<STDIN>) {
                   while (s|({.*}.*){.*}|$1 |) {}
                   s|{.*}| |;
                   if (s|{.*| |) {
                       $front = $_;
                       while (<STDIN>) {
                           if (/}/) {  # end of comment?
                               redo LINE;

           "redo" cannot be used to retry a block that returns a value such as
           "eval {}", "sub {}", or "do {}", and should not be used to exit a
           grep() or map() operation.

           Note that a block by itself is semantically identical to a loop
           that executes once.  Thus "redo" inside such a block will
           effectively turn it into a looping construct.

           See also "continue" for an illustration of how "last", "next", and
           "redo" work.

       ref EXPR
       ref Returns a non-empty string if EXPR is a reference, the empty string
           otherwise. If EXPR is not specified, $_ will be used.  The value
           returned depends on the type of thing the reference is a reference
           to.  Builtin types include:


           If the referenced object has been blessed into a package, then that
           package name is returned instead.  You can think of "ref" as a
           "typeof" operator.

               if (ref($r) eq "HASH") {
                   print "r is a reference to a hash.\n";
               unless (ref($r)) {

       rename OLDNAME,NEWNAME
           Changes the name of a file; an existing file NEWNAME will be
           clobbered.  Returns true for success, false otherwise.

           Behavior of this function varies wildly depending on your system
           implementation.  For example, it will usually not work across file
           system boundaries, even though the system mv command sometimes
           compensates for this.  Other restrictions include whether it works
           on directories, open files, or pre-existing files.  Check perlport
           and either the rename(2) manpage or equivalent system documentation
           for details.

           For a platform independent "move" function look at the File::Copy

       require VERSION
       require EXPR
           Demands a version of Perl specified by VERSION, or demands some
           semantics specified by EXPR or by $_ if EXPR is not supplied.

           VERSION may be either a numeric argument such as 5.006, which will
           be compared to $], or a literal of the form v5.6.1, which will be
           compared to $^V (aka $PERL_VERSION).  An exception is raised if
           VERSION is greater than the version of the current Perl
           interpreter.  Compare with "use", which can do a similar check at
           compile time.

           Specifying VERSION as a literal of the form v5.6.1 should generally
           be avoided, because it leads to misleading error messages under
           earlier versions of Perl that do not support this syntax.  The
           equivalent numeric version should be used instead.

               require v5.6.1;     # run time version check
               require 5.6.1;      # ditto
               require 5.006_001;  # ditto; preferred for backwards compatibility

           Otherwise, "require" demands that a library file be included if it
           hasn't already been included.  The file is included via the do-FILE
           mechanism, which is essentially just a variety of "eval" with the
           caveat that lexical variables in the invoking script will be
           invisible to the included code.  Has semantics similar to the
           following subroutine:

               sub require {
                  my ($filename) = @_;
                  if (exists $INC{$filename}) {
                      return 1 if $INC{$filename};
                      die "Compilation failed in require";
                  my ($realfilename,$result);
                  ITER: {
                      foreach $prefix (@INC) {

                  } elsif (!$result) {
                      delete $INC{$filename};
                      die "$filename did not return true value";
                  } else {
                      return $result;

           Note that the file will not be included twice under the same
           specified name.

           The file must return true as the last statement to indicate
           successful execution of any initialization code, so it's customary
           to end such a file with "1;" unless you're sure it'll return true
           otherwise.  But it's better just to put the "1;", in case you add
           more statements.

           If EXPR is a bareword, the require assumes a ".pm" extension and
           replaces "::" with "/" in the filename for you, to make it easy to
           load standard modules.  This form of loading of modules does not
           risk altering your namespace.

           In other words, if you try this:

                   require Foo::Bar;     # a splendid bareword

           The require function will actually look for the "Foo/" file
           in the directories specified in the @INC array.

           But if you try this:

                   $class = 'Foo::Bar';
                   require $class;       # $class is not a bareword
                   require "Foo::Bar";   # not a bareword because of the ""

           The require function will look for the "Foo::Bar" file in the @INC
           array and will complain about not finding "Foo::Bar" there.  In
           this case you can do:

                   eval "require $class";

           Now that you understand how "require" looks for files with a
           bareword argument, there is a little extra functionality going on
           behind the scenes.  Before "require" looks for a ".pm" extension,
           it will first look for a similar filename with a ".pmc" extension.
           If this file is found, it will be loaded in place of any file
           ending in a ".pm" extension.

           You can also insert hooks into the import facility by putting Perl
           code directly into the @INC array.  There are three forms of hooks:
           subroutine references, array references, and blessed objects.

           Subroutine references are the simplest case.  When the inclusion
               is a filehandle, then the subroutine will be called to act as a
               simple source filter, with the line as read in $_.  Again,
               return 1 for each valid line, and 0 after all lines have been

           3.  Optional state for the subroutine. The state is passed in as
               $_[1]. A reference to the subroutine itself is passed in as

           If an empty list, "undef", or nothing that matches the first 3
           values above is returned, then "require" looks at the remaining
           elements of @INC.  Note that this filehandle must be a real
           filehandle (strictly a typeglob or reference to a typeglob, whether
           blessed or unblessed); tied filehandles will be ignored and
           processing will stop there.

           If the hook is an array reference, its first element must be a
           subroutine reference.  This subroutine is called as above, but the
           first parameter is the array reference.  This lets you indirectly
           pass arguments to the subroutine.

           In other words, you can write:

               push @INC, \&my_sub;
               sub my_sub {
                   my ($coderef, $filename) = @_;  # $coderef is \&my_sub


               push @INC, [ \&my_sub, $x, $y, ... ];
               sub my_sub {
                   my ($arrayref, $filename) = @_;
                   # Retrieve $x, $y, ...
                   my @parameters = @$arrayref[1..$#$arrayref];

           If the hook is an object, it must provide an INC method that will
           be called as above, the first parameter being the object itself.
           (Note that you must fully qualify the sub's name, as unqualified
           "INC" is always forced into package "main".)  Here is a typical
           code layout:

               # In
               package Foo;
               sub new { ... }
               sub Foo::INC {
                   my ($self, $filename) = @_;

               # In the main program

           allowed for ranges).  All variables and arrays beginning with one
           of those letters are reset to their pristine state.  If the
           expression is omitted, one-match searches ("?pattern?") are reset
           to match again.  Only resets variables or searches in the current
           package.  Always returns 1.  Examples:

               reset 'X';      # reset all X variables
               reset 'a-z';    # reset lower case variables
               reset;          # just reset ?one-time? searches

           Resetting "A-Z" is not recommended because you'll wipe out your
           @ARGV and @INC arrays and your %ENV hash.  Resets only package
           variables; lexical variables are unaffected, but they clean
           themselves up on scope exit anyway, so you'll probably want to use
           them instead.  See "my".

       return EXPR
           Returns from a subroutine, "eval", or "do FILE" with the value
           given in EXPR.  Evaluation of EXPR may be in list, scalar, or void
           context, depending on how the return value will be used, and the
           context may vary from one execution to the next (see "wantarray").
           If no EXPR is given, returns an empty list in list context, the
           undefined value in scalar context, and (of course) nothing at all
           in void context.

           (In the absence of an explicit "return", a subroutine, eval, or do
           FILE automatically returns the value of the last expression

       reverse LIST
           In list context, returns a list value consisting of the elements of
           LIST in the opposite order.  In scalar context, concatenates the
           elements of LIST and returns a string value with all characters in
           the opposite order.

               print join(", ", reverse "world", "Hello"); # Hello, world

               print scalar reverse "dlrow ,", "olleH";    # Hello, world

           Used without arguments in scalar context, reverse() reverses $_.

               $_ = "dlrow ,olleH";
               print reverse;                              # No output, list context
               print scalar reverse;                       # Hello, world

           Note that reversing an array to itself (as in "@a = reverse @a")
           will preserve non-existent elements whenever possible, i.e., for
           non magical arrays or tied arrays with "EXISTS" and "DELETE"

           This operator is also handy for inverting a hash, although there
           are some caveats.  If a value is duplicated in the original hash,
           only one of those can be represented as a key in the inverted hash.
           last occurrence of SUBSTR in STR.  If POSITION is specified,
           returns the last occurrence beginning at or before that position.

       rmdir FILENAME
           Deletes the directory specified by FILENAME if that directory is
           empty.  If it succeeds it returns true; otherwise it returns false
           and sets $! (errno).  If FILENAME is omitted, uses $_.

           To remove a directory tree recursively ("rm -rf" on Unix) look at
           the "rmtree" function of the File::Path module.

           The substitution operator.  See "Regexp Quote-Like Operators" in

       say FILEHANDLE
       say LIST
       say Just like "print", but implicitly appends a newline.  "say LIST" is
           simply an abbreviation for "{ local $\ = "\n"; print LIST }".  To
           use FILEHANDLE without a LIST to print the contents of $_ to it,
           you must use a real filehandle like "FH", not an indirect one like

           This keyword is available only when the "say" feature is enabled;
           see feature.  Alternately, include a "use v5.10" or later to the
           current scope.

       scalar EXPR
           Forces EXPR to be interpreted in scalar context and returns the
           value of EXPR.

               @counts = ( scalar @a, scalar @b, scalar @c );

           There is no equivalent operator to force an expression to be
           interpolated in list context because in practice, this is never
           needed.  If you really wanted to do so, however, you could use the
           construction "@{[ (some expression) ]}", but usually a simple
           "(some expression)" suffices.

           Because "scalar" is a unary operator, if you accidentally use a
           parenthesized list for the EXPR, this behaves as a scalar comma
           expression, evaluating all but the last element in void context and
           returning the final element evaluated in scalar context.  This is
           seldom what you want.

           The following single statement:

               print uc(scalar(&foo,$bar)),$baz;

           is the moral equivalent of these two:


           "SEEK_END" (start of the file, current position, end of the file)
           from the Fcntl module.  Returns 1 on success, false otherwise.

           Note the in bytes: even if the filehandle has been set to operate
           on characters (for example by using the ":encoding(utf8)" open
           layer), tell() will return byte offsets, not character offsets
           (because implementing that would render seek() and tell() rather

           If you want to position the file for "sysread" or "syswrite", don't
           use "seek", because buffering makes its effect on the file's read-
           write position unpredictable and non-portable.  Use "sysseek"

           Due to the rules and rigors of ANSI C, on some systems you have to
           do a seek whenever you switch between reading and writing.  Amongst
           other things, this may have the effect of calling stdio's
           clearerr(3).  A WHENCE of 1 ("SEEK_CUR") is useful for not moving
           the file position:


           This is also useful for applications emulating "tail -f".  Once you
           hit EOF on your read and then sleep for a while, you (probably)
           have to stick in a dummy seek() to reset things.  The "seek"
           doesn't change the position, but it does clear the end-of-file
           condition on the handle, so that the next "<FILE>" makes Perl try
           again to read something.  (We hope.)

           If that doesn't work (some I/O implementations are particularly
           cantankerous), you might need something like this:

               for (;;) {
                   for ($curpos = tell(FILE); $_ = <FILE>;
                        $curpos = tell(FILE)) {
                       # search for some stuff and put it into files
                   seek(FILE, $curpos, 0);

       seekdir DIRHANDLE,POS
           Sets the current position for the "readdir" routine on DIRHANDLE.
           POS must be a value returned by "telldir".  "seekdir" also has the
           same caveats about possible directory compaction as the
           corresponding system library routine.

       select FILEHANDLE
           Returns the currently selected filehandle.  If FILEHANDLE is
           supplied, sets the new current default filehandle for output.  This
           has two effects: first, a "write" or a "print" without a filehandle
           default to this FILEHANDLE.  Second, references to variables
           related to output will refer to this output channel.
               $oldfh = select(STDERR); $| = 1; select($oldfh);

           Some programmers may prefer to think of filehandles as objects with
           methods, preferring to write the last example as:

               use IO::Handle;

           This calls the select(2) syscall with the bit masks specified,
           which can be constructed using "fileno" and "vec", along these

               $rin = $win = $ein = '';
               vec($rin,fileno(STDIN),1) = 1;
               vec($win,fileno(STDOUT),1) = 1;
               $ein = $rin | $win;

           If you want to select on many filehandles, you may wish to write a
           subroutine like this:

               sub fhbits {
                   my(@fhlist) = split(' ',$_[0]);
                   for (@fhlist) {
                       vec($bits,fileno($_),1) = 1;
               $rin = fhbits('STDIN TTY SOCK');

           The usual idiom is:

               ($nfound,$timeleft) =
                 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

           or to block until something becomes ready just do this

               $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);

           Most systems do not bother to return anything useful in $timeleft,
           so calling select() in scalar context just returns $nfound.

           Any of the bit masks can also be undef.  The timeout, if specified,
           is in seconds, which may be fractional.  Note: not all
           implementations are capable of returning the $timeleft.  If not,
           they always return $timeleft equal to the supplied $timeout.

           You can effect a sleep of 250 milliseconds this way:

               select(undef, undef, undef, 0.25);

           Note that whether "select" gets restarted after signals (say,
           SIGALRM) is implementation-dependent.  See also perlport for notes
           <FH>) with "select", except as permitted by POSIX, and even then
           only on POSIX systems.  You have to use "sysread" instead.

       semctl ID,SEMNUM,CMD,ARG
           Calls the System V IPC function semctl(2).  You'll probably have to

               use IPC::SysV;

           first to get the correct constant definitions.  If CMD is IPC_STAT
           or GETALL, then ARG must be a variable that will hold the returned
           semid_ds structure or semaphore value array.  Returns like "ioctl":
           the undefined value for error, ""0 but true"" for zero, or the
           actual return value otherwise.  The ARG must consist of a vector of
           native short integers, which may be created with
           "pack("s!",(0)x$nsem)".  See also "SysV IPC" in perlipc,
           "IPC::SysV", "IPC::Semaphore" documentation.

       semget KEY,NSEMS,FLAGS
           Calls the System V IPC function semget(2).  Returns the semaphore
           id, or the undefined value on error.  See also "SysV IPC" in
           perlipc, "IPC::SysV", "IPC::SysV::Semaphore" documentation.

       semop KEY,OPSTRING
           Calls the System V IPC function semop(2) for semaphore operations
           such as signalling and waiting.  OPSTRING must be a packed array of
           semop structures.  Each semop structure can be generated with
           "pack("s!3", $semnum, $semop, $semflag)".  The length of OPSTRING
           implies the number of semaphore operations.  Returns true if
           successful, false on error.  As an example, the following code
           waits on semaphore $semnum of semaphore id $semid:

               $semop = pack("s!3", $semnum, -1, 0);
               die "Semaphore trouble: $!\n" unless semop($semid, $semop);

           To signal the semaphore, replace "-1" with 1.  See also "SysV IPC"
           in perlipc, "IPC::SysV", and "IPC::SysV::Semaphore" documentation.

       send SOCKET,MSG,FLAGS
           Sends a message on a socket.  Attempts to send the scalar MSG to
           the SOCKET filehandle.  Takes the same flags as the system call of
           the same name.  On unconnected sockets, you must specify a
           destination to send to, in which case it does a sendto(2) syscall.
           Returns the number of characters sent, or the undefined value on
           error.  The sendmsg(2) syscall is currently unimplemented.  See
           "UDP: Message Passing" in perlipc for examples.

           Note the characters: depending on the status of the socket, either
           (8-bit) bytes or characters are sent.  By default all sockets
           operate on bytes, but for example if the socket has been changed
           using binmode() to operate with the ":encoding(utf8)" I/O layer
           (see "open", or the "open" pragma, open), the I/O will operate on
           UTF-8 encoded Unicode characters, not bytes.  Similarly for the
           Sets the current priority for a process, a process group, or a
           user.  (See setpriority(2).)  Raises an exception when used on a
           machine that doesn't implement setpriority(2).

           Sets the socket option requested.  Returns "undef" on error.  Use
           integer constants provided by the "Socket" module for LEVEL and
           OPNAME.  Values for LEVEL can also be obtained from getprotobyname.
           OPTVAL might either be a packed string or an integer.  An integer
           OPTVAL is shorthand for pack("i", OPTVAL).

           An example disabling Nagle's algorithm on a socket:

               use Socket qw(IPPROTO_TCP TCP_NODELAY);
               setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);

       shift ARRAY
       shift EXPR
           Shifts the first value of the array off and returns it, shortening
           the array by 1 and moving everything down.  If there are no
           elements in the array, returns the undefined value.  If ARRAY is
           omitted, shifts the @_ array within the lexical scope of
           subroutines and formats, and the @ARGV array outside a subroutine
           and also within the lexical scopes established by the "eval
           STRING", "BEGIN {}", "INIT {}", "CHECK {}", "UNITCHECK {}", and
           "END {}" constructs.

           Starting with Perl 5.14, "shift" can take a scalar EXPR, which must
           hold a reference to an unblessed array.  The argument will be
           dereferenced automatically.  This aspect of "shift" is considered
           highly experimental.  The exact behaviour may change in a future
           version of Perl.

           See also "unshift", "push", and "pop".  "shift" and "unshift" do
           the same thing to the left end of an array that "pop" and "push" do
           to the right end.

       shmctl ID,CMD,ARG
           Calls the System V IPC function shmctl.  You'll probably have to

               use IPC::SysV;

           first to get the correct constant definitions.  If CMD is
           "IPC_STAT", then ARG must be a variable that will hold the returned
           "shmid_ds" structure.  Returns like ioctl: "undef" for error; "0
           but true" for zero; and the actual return value otherwise.  See
           also "SysV IPC" in perlipc and "IPC::SysV" documentation.

       shmget KEY,SIZE,FLAGS
           Calls the System V IPC function shmget.  Returns the shared memory
           segment id, or "undef" on error.  See also "SysV IPC" in perlipc
           and "IPC::SysV" documentation.

       shutdown SOCKET,HOW
           Shuts down a socket connection in the manner indicated by HOW,
           which has the same interpretation as in the syscall of the same

               shutdown(SOCKET, 0);    # I/we have stopped reading data
               shutdown(SOCKET, 1);    # I/we have stopped writing data
               shutdown(SOCKET, 2);    # I/we have stopped using this socket

           This is useful with sockets when you want to tell the other side
           you're done writing but not done reading, or vice versa.  It's also
           a more insistent form of close because it also disables the file
           descriptor in any forked copies in other processes.

           Returns 1 for success; on error, returns "undef" if the first
           argument is not a valid filehandle, or returns 0 and sets $! for
           any other failure.

       sin EXPR
       sin Returns the sine of EXPR (expressed in radians).  If EXPR is
           omitted, returns sine of $_.

           For the inverse sine operation, you may use the "Math::Trig::asin"
           function, or use this relation:

               sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }

       sleep EXPR
           Causes the script to sleep for (integer) EXPR seconds, or forever
           if no argument is given.  Returns the integer number of seconds
           actually slept.

           May be interrupted if the process receives a signal such as

               eval {
                   local $SIG{ALARM} = sub { die "Alarm!\n" };
               die $@ unless $@ eq "Alarm!\n";

           You probably cannot mix "alarm" and "sleep" calls, because "sleep"
           is often implemented using "alarm".

           On some older systems, it may sleep up to a full second less than
           what you requested, depending on how it counts seconds.  Most
           modern systems always sleep the full amount.  They may appear to
           sleep longer than that, however, because your process might not be
           scheduled right away in a busy multitasking system.

           For delays of finer granularity than one second, the Time::HiRes
           module (from CPAN, and starting from Perl 5.8 part of the standard
           distribution) provides usleep().  You may also use Perl's four-
           Client/Server Communication" in perlipc.

           On systems that support a close-on-exec flag on files, the flag
           will be set for the newly opened file descriptor, as determined by
           the value of $^F.  See "$^F" in perlvar.

           Creates an unnamed pair of sockets in the specified domain, of the
           specified type.  DOMAIN, TYPE, and PROTOCOL are specified the same
           as for the syscall of the same name.  If unimplemented, raises an
           exception.  Returns true if successful.

           On systems that support a close-on-exec flag on files, the flag
           will be set for the newly opened file descriptors, as determined by
           the value of $^F.  See "$^F" in perlvar.

           Some systems defined "pipe" in terms of "socketpair", in which a
           call to "pipe(Rdr, Wtr)" is essentially:

               use Socket;
               socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
               shutdown(Rdr, 1);        # no more writing for reader
               shutdown(Wtr, 0);        # no more reading for writer

           See perlipc for an example of socketpair use.  Perl 5.8 and later
           will emulate socketpair using IP sockets to localhost if your
           system implements sockets but not socketpair.

       sort SUBNAME LIST
       sort BLOCK LIST
       sort LIST
           In list context, this sorts the LIST and returns the sorted list
           value.  In scalar context, the behaviour of "sort()" is undefined.

           If SUBNAME or BLOCK is omitted, "sort"s in standard string
           comparison order.  If SUBNAME is specified, it gives the name of a
           subroutine that returns an integer less than, equal to, or greater
           than 0, depending on how the elements of the list are to be
           ordered.  (The "<=>" and "cmp" operators are extremely useful in
           such routines.)  SUBNAME may be a scalar variable name
           (unsubscripted), in which case the value provides the name of (or a
           reference to) the actual subroutine to use.  In place of a SUBNAME,
           you can provide a BLOCK as an anonymous, in-line sort subroutine.

           If the subroutine's prototype is "($$)", the elements to be
           compared are passed by reference in @_, as for a normal subroutine.
           This is slower than unprototyped subroutines, where the elements to
           be compared are passed into the subroutine as the package global
           variables $a and $b (see example below).  Note that in the latter
           case, it is usually highly counter-productive to declare $a and $b
           as lexicals.

           The values to be compared are always passed by reference and should
           not be modified.

           Perl 5.6 and earlier used a quicksort algorithm to implement sort.
           That algorithm was not stable, so could go quadratic.  (A stable
           sort preserves the input order of elements that compare equal.
           Although quicksort's run time is O(NlogN) when averaged over all
           arrays of length N, the time can be O(N**2), quadratic behavior,
           for some inputs.)  In 5.7, the quicksort implementation was
           replaced with a stable mergesort algorithm whose worst-case
           behavior is O(NlogN).  But benchmarks indicated that for some
           inputs, on some platforms, the original quicksort was faster.  5.8
           has a sort pragma for limited control of the sort.  Its rather
           blunt control of the underlying algorithm may not persist into
           future Perls, but the ability to characterize the input or output
           in implementation independent ways quite probably will.  See the
           sort pragma.


               # sort lexically
               @articles = sort @files;

               # same thing, but with explicit sort routine
               @articles = sort {$a cmp $b} @files;

               # now case-insensitively
               @articles = sort {uc($a) cmp uc($b)} @files;

               # same thing in reversed order
               @articles = sort {$b cmp $a} @files;

               # sort numerically ascending
               @articles = sort {$a <=> $b} @files;

               # sort numerically descending
               @articles = sort {$b <=> $a} @files;

               # this sorts the %age hash by value instead of key
               # using an in-line function
               @eldest = sort { $age{$b} <=> $age{$a} } keys %age;

               # sort using explicit subroutine name
               sub byage {
               $age{$a} <=> $age{$b};  # presuming numeric
               @sortedclass = sort byage @class;

               sub backwards { $b cmp $a }
               @harry  = qw(dog cat x Cain Abel);
               @george = qw(gone chased yz Punished Axed);
               print sort @harry;
                   # prints AbelCaincatdogx
               print sort backwards @harry;
                   # prints xdogcatCainAbel

               # same thing, but much more efficiently;
               # we'll build auxiliary indices instead
               # for speed
               my @nums = @caps = ();
               for (@old) {
                   push @nums, ( /=(\d+)/ ? $1 : undef );
                   push @caps, uc($_);

               my @new = @old[ sort {
                   $nums[$b] <=> $nums[$a]
                   $caps[$a] cmp $caps[$b]
                   } 0..$#old

               # same thing, but without any temps
               @new = map { $_->[0] }
                      sort { $b->[1] <=> $a->[1]
                      $a->[2] cmp $b->[2]
               } map { [$_, /=(\d+)/, uc($_)] } @old;

               # using a prototype allows you to use any comparison subroutine
               # as a sort subroutine (including other package's subroutines)
               package other;
               sub backwards ($$) { $_[1] cmp $_[0]; }  # $a and $b are not set here

               package main;
               @new = sort other::backwards @old;

               # guarantee stability, regardless of algorithm
               use sort 'stable';
               @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;

               # force use of mergesort (not portable outside Perl 5.8)
               use sort '_mergesort';  # note discouraging _
               @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;

           Warning: syntactical care is required when sorting the list
           returned from a function. If you want to sort the list returned by
           the function call "find_records(@key)", you can use:

               @contact = sort { $a cmp $b } find_records @key;
               @contact = sort +find_records(@key);
               @contact = sort &find_records(@key);
               @contact = sort(find_records(@key));

           If instead you want to sort the array @key with the comparison
           routine "find_records()" then you can use:

               @contact = sort { find_records() } @key;
               @contact = sort find_records(@key);
               @articles = sort {$FooPack::b <=> $FooPack::a} @files;

           The comparison function is required to behave.  If it returns
           inconsistent results (sometimes saying $x[1] is less than $x[2] and
           sometimes saying the opposite, for example) the results are not

           Because "<=>" returns "undef" when either operand is "NaN" (not-a-
           number), and also because "sort" raises an exception unless the
           result of a comparison is defined, be careful when sorting with a
           comparison function like "$a <=> $b" any lists that might contain a
           "NaN".  The following example takes advantage that "NaN != NaN" to
           eliminate any "NaN"s from the input list.

               @result = sort { $a <=> $b } grep { $_ == $_ } @input;

       splice ARRAY or EXPR,OFFSET
       splice ARRAY or EXPR
           Removes the elements designated by OFFSET and LENGTH from an array,
           and replaces them with the elements of LIST, if any.  In list
           context, returns the elements removed from the array.  In scalar
           context, returns the last element removed, or "undef" if no
           elements are removed.  The array grows or shrinks as necessary.  If
           OFFSET is negative then it starts that far from the end of the
           array.  If LENGTH is omitted, removes everything from OFFSET
           onward.  If LENGTH is negative, removes the elements from OFFSET
           onward except for -LENGTH elements at the end of the array.  If
           both OFFSET and LENGTH are omitted, removes everything. If OFFSET
           is past the end of the array, Perl issues a warning, and splices at
           the end of the array.

           The following equivalences hold (assuming "$[ == 0 and $#a >= $i" )

               push(@a,$x,$y)      splice(@a,@a,0,$x,$y)
               pop(@a)             splice(@a,-1)
               shift(@a)           splice(@a,0,1)
               unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)
               $a[$i] = $y         splice(@a,$i,1,$y)

           Example, assuming array lengths are passed before arrays:

               sub aeq {  # compare two list values
                   my(@a) = splice(@_,0,shift);
                   my(@b) = splice(@_,0,shift);
                   return 0 unless @a == @b;  # same len?
                   while (@a) {
                       return 0 if pop(@a) ne pop(@b);
                   return 1;
               if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }

           trailing ones are deleted.  (If all fields are empty, they are
           considered to be trailing.)

           In scalar context, returns the number of fields found.

           If EXPR is omitted, splits the $_ string.  If PATTERN is also
           omitted, splits on whitespace (after skipping any leading
           whitespace).  Anything matching PATTERN is taken to be a delimiter
           separating the fields.  (Note that the delimiter may be longer than
           one character.)

           If LIMIT is specified and positive, it represents the maximum
           number of fields the EXPR will be split into, though the actual
           number of fields returned depends on the number of times PATTERN
           matches within EXPR.  If LIMIT is unspecified or zero, trailing
           null fields are stripped (which potential users of "pop" would do
           well to remember).  If LIMIT is negative, it is treated as if an
           arbitrarily large LIMIT had been specified.  Note that splitting an
           EXPR that evaluates to the empty string always returns the empty
           list, regardless of the LIMIT specified.

           A pattern matching the empty string (not to be confused with an
           empty pattern "//", which is just one member of the set of patterns
           matching the empty string), splits EXPR into individual characters.
           For example:

               print join(':', split(/ */, 'hi there')), "\n";

           produces the output 'h:i:t:h:e:r:e'.

           As a special case for "split", the empty pattern "//" specifically
           matches the empty string; this is not be confused with the normal
           use of an empty pattern to mean the last successful match.  So to
           split a string into individual characters, the following:

               print join(':', split(//, 'hi there')), "\n";

           produces the output 'h:i: :t:h:e:r:e'.

           Empty leading fields are produced when there are positive-width
           matches at the beginning of the string; a zero-width match at the
           beginning of the string does not produce an empty field. For

              print join(':', split(/(?=\w)/, 'hi there!'));

           produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on
           the other hand, are produced when there is a match at the end of
           the string (and when LIMIT is given and is not 0), regardless of
           the length of the match.  For example:

              print join(':', split(//,   'hi there!', -1)), "\n";
              print join(':', split(/\W/, 'hi there!', -1)), "\n";

           If the PATTERN contains parentheses, additional list elements are
           created from each matching substring in the delimiter.

               split(/([,-])/, "1-10,20", 3);

           produces the list value

               (1, '-', 10, ',', 20)

           If you had the entire header of a normal Unix email message in
           $header, you could split it up into fields and their values this

               $header =~ s/\n(?=\s)//g;  # fix continuation lines
               %hdrs   =  (UNIX_FROM => split /^(\S*?):\s*/m, $header);

           The pattern "/PATTERN/" may be replaced with an expression to
           specify patterns that vary at runtime.  (To do runtime compilation
           only once, use "/$variable/o".)

           As a special case, specifying a PATTERN of space (' ') will split
           on white space just as "split" with no arguments does.  Thus,
           "split(' ')" can be used to emulate awk's default behavior, whereas
           "split(/ /)" will give you as many initial null fields (empty
           string) as there are leading spaces.  A "split" on "/\s+/" is like
           a "split(' ')" except that any leading whitespace produces a null
           first field.  A "split" with no arguments really does a
           "split(' ', $_)" internally.

           A PATTERN of "/^/" is treated as if it were "/^/m", since it isn't
           much use otherwise.


               open(PASSWD, '/etc/passwd');
               while (<PASSWD>) {
                   ($login, $passwd, $uid, $gid,
                    $gcos, $home, $shell) = split(/:/);

           As with regular pattern matching, any capturing parentheses that
           are not matched in a "split()" will be set to "undef" when

               @fields = split /(A)|B/, "1A2B3";
               # @fields is (1, 'A', 2, undef, 3)

       sprintf FORMAT, LIST
           Returns a string formatted by the usual "printf" conventions of the
           C library function "sprintf".  See below for more details and see
           sprintf(3) or printf(3) on your system for an explanation of the
           and even then only standard modifiers are allowed.  Non-standard
           extensions in your local sprintf(3) are therefore unavailable from

           Unlike "printf", "sprintf" does not do what you probably mean when
           you pass it an array as your first argument. The array is given
           scalar context, and instead of using the 0th element of the array
           as the format, Perl will use the count of elements in the array as
           the format, which is almost never useful.

           Perl's "sprintf" permits the following universally-known

              %%    a percent sign
              %c    a character with the given number
              %s    a string
              %d    a signed integer, in decimal
              %u    an unsigned integer, in decimal
              %o    an unsigned integer, in octal
              %x    an unsigned integer, in hexadecimal
              %e    a floating-point number, in scientific notation
              %f    a floating-point number, in fixed decimal notation
              %g    a floating-point number, in %e or %f notation

           In addition, Perl permits the following widely-supported

              %X    like %x, but using upper-case letters
              %E    like %e, but using an upper-case "E"
              %G    like %g, but with an upper-case "E" (if applicable)
              %b    an unsigned integer, in binary
              %B    like %b, but using an upper-case "B" with the # flag
              %p    a pointer (outputs the Perl value's address in hexadecimal)
              %n    special: *stores* the number of characters output so far
                   into the next variable in the parameter list

           Finally, for backward (and we do mean "backward") compatibility,
           Perl permits these unnecessary but widely-supported conversions:

              %i    a synonym for %d
              %D    a synonym for %ld
              %U    a synonym for %lu
              %O    a synonym for %lo
              %F    a synonym for %f

           Note that the number of exponent digits in the scientific notation
           produced by %e, %E, %g and %G for numbers with the modulus of the
           exponent less than 100 is system-dependent: it may be three or less
           (zero-padded as necessary).  In other words, 1.23 times ten to the
           99th may be either "1.23e99" or "1.23e099".

           Between the "%" and the format letter, you may specify several
           additional attributes controlling the interpretation of the format.
           In order, these are:
                  space   prefix non-negative number with a space
                  +       prefix non-negative number with a plus sign
                  -       left-justify within the field
                  0       use zeros, not spaces, to right-justify
                  #       ensure the leading "0" for any octal,
                          prefix non-zero hexadecimal with "0x" or "0X",
                          prefix non-zero binary with "0b" or "0B"

               For example:

                 printf '<% d>',  12;   # prints "< 12>"
                 printf '<%+d>',  12;   # prints "<+12>"
                 printf '<%6s>',  12;   # prints "<    12>"
                 printf '<%-6s>', 12;   # prints "<12    >"
                 printf '<%06s>', 12;   # prints "<000012>"
                 printf '<%#o>',  12;   # prints "<014>"
                 printf '<%#x>',  12;   # prints "<0xc>"
                 printf '<%#X>',  12;   # prints "<0XC>"
                 printf '<%#b>',  12;   # prints "<0b1100>"
                 printf '<%#B>',  12;   # prints "<0B1100>"

               When a space and a plus sign are given as the flags at once, a
               plus sign is used to prefix a positive number.

                 printf '<%+ d>', 12;   # prints "<+12>"
                 printf '<% +d>', 12;   # prints "<+12>"

               When the # flag and a precision are given in the %o conversion,
               the precision is incremented if it's necessary for the leading

                 printf '<%#.5o>', 012;      # prints "<00012>"
                 printf '<%#.5o>', 012345;   # prints "<012345>"
                 printf '<%#.0o>', 0;        # prints "<0>"

           vector flag
               This flag tells Perl to interpret the supplied string as a
               vector of integers, one for each character in the string. Perl
               applies the format to each integer in turn, then joins the
               resulting strings with a separator (a dot "." by default). This
               can be useful for displaying ordinal values of characters in
               arbitrary strings:

                 printf "%vd", "AB\x{100}";           # prints "65.66.256"
                 printf "version is v%vd\n", $^V;     # Perl's version

               Put an asterisk "*" before the "v" to override the string to
               use to separate the numbers:

                 printf "address is %*vX\n", ":", $addr;   # IPv6 address
                 printf "bits are %0*v8b\n", " ", $bits;   # random bitstring

               You can also explicitly specify the argument number to use for
               the join string using something like "*2$v"; for example:
                 printf "<%*2$s>", "a", 6; # prints "<     a>"
                 printf "<%2s>", "long";   # prints "<long>" (does not truncate)

               If a field width obtained through "*" is negative, it has the
               same effect as the "-" flag: left-justification.

           precision, or maximum width
               You can specify a precision (for numeric conversions) or a
               maximum width (for string conversions) by specifying a "."
               followed by a number.  For floating-point formats except "g"
               and "G", this specifies how many places right of the decimal
               point to show (the default being 6).  For example:

                 # these examples are subject to system-specific variation
                 printf '<%f>', 1;    # prints "<1.000000>"
                 printf '<%.1f>', 1;  # prints "<1.0>"
                 printf '<%.0f>', 1;  # prints "<1>"
                 printf '<%e>', 10;   # prints "<1.000000e+01>"
                 printf '<%.1e>', 10; # prints "<1.0e+01>"

               For "g" and "G", this specifies the maximum number of digits to
               show, including those prior to the decimal point and those
               after it; for example:

                 # These examples are subject to system-specific variation.
                 printf '<%g>', 1;        # prints "<1>"
                 printf '<%.10g>', 1;     # prints "<1>"
                 printf '<%g>', 100;      # prints "<100>"
                 printf '<%.1g>', 100;    # prints "<1e+02>"
                 printf '<%.2g>', 100.01; # prints "<1e+02>"
                 printf '<%.5g>', 100.01; # prints "<100.01>"
                 printf '<%.4g>', 100.01; # prints "<100>"

               For integer conversions, specifying a precision implies that
               the output of the number itself should be zero-padded to this
               width, where the 0 flag is ignored:

                 printf '<%.6d>', 1;      # prints "<000001>"
                 printf '<%+.6d>', 1;     # prints "<+000001>"
                 printf '<%-10.6d>', 1;   # prints "<000001    >"
                 printf '<%10.6d>', 1;    # prints "<    000001>"
                 printf '<%010.6d>', 1;   # prints "<    000001>"
                 printf '<%+10.6d>', 1;   # prints "<   +000001>"

                 printf '<%.6x>', 1;      # prints "<000001>"
                 printf '<%#.6x>', 1;     # prints "<0x000001>"
                 printf '<%-10.6x>', 1;   # prints "<000001    >"
                 printf '<%10.6x>', 1;    # prints "<    000001>"
                 printf '<%010.6x>', 1;   # prints "<    000001>"
                 printf '<%#10.6x>', 1;   # prints "<  0x000001>"

               For string conversions, specifying a precision truncates the
               string to fit the specified width:

                 printf '<%.*s>',  7, "string";   # prints "<string>"
                 printf '<%.*s>',  3, "string";   # prints "<str>"
                 printf '<%.*s>',  0, "string";   # prints "<>"
                 printf '<%.*s>', -1, "string";   # prints "<string>"

                 printf '<%.*d>',  1, 0;   # prints "<0>"
                 printf '<%.*d>',  0, 0;   # prints "<>"
                 printf '<%.*d>', -1, 0;   # prints "<0>"

               You cannot currently get the precision from a specified number,
               but it is intended that this will be possible in the future,
               for example using ".*2$":

                 printf "<%.*2$x>", 1, 6;   # INVALID, but in future will print "<000001>"

               For numeric conversions, you can specify the size to interpret
               the number as using "l", "h", "V", "q", "L", or "ll". For
               integer conversions ("d u o x X b i D U O"), numbers are
               usually assumed to be whatever the default integer size is on
               your platform (usually 32 or 64 bits), but you can override
               this to use instead one of the standard C types, as supported
               by the compiler used to build Perl:

                  hh          interpret integer as C type "char" or "unsigned char"
                              on Perl 5.14 or later
                  h           interpret integer as C type "short" or "unsigned short"
                  j           intepret integer as C type "intmax_t" on Perl 5.14
                              or later, and only with a C99 compiler (unportable)
                  l           interpret integer as C type "long" or "unsigned long"
                  q, L, or ll interpret integer as C type "long long", "unsigned long long",
                              or "quad" (typically 64-bit integers)
                  t           intepret integer as C type "ptrdiff_t" on Perl 5.14 or later
                  z           intepret integer as C type "size_t" on Perl 5.14 or later

               As of 5.14, none of these raises an exception if they are not
               supported on your platform.  However, if warnings are enabled,
               a warning of the "printf" warning class is issued on an
               unsupported conversion flag.  Should you instead prefer an
               exception, do this:

                   use warnings FATAL => "printf";

               If you would like to know about a version dependency before you
               start running the program, put something like this at its top:

                   use 5.014;  # for hh/j/t/z/ printf modifiers

               You can find out whether your Perl supports quads via Config:

                   use Config;
                   if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
                       print "Nice quads!\n";

               default floating-point size to use on your platform via Config:

                   use Config;
                   if ($Config{uselongdouble} eq "define") {
                       print "long doubles by default\n";

               It can also be that long doubles and doubles are the same

                       use Config;
                       ($Config{doublesize} == $Config{longdblsize}) &&
                               print "doubles are long doubles\n";

               The size specifier "V" has no effect for Perl code, but is
               supported for compatibility with XS code.  It means "use the
               standard size for a Perl integer or floating-point number",
               which is the default.

           order of arguments
               Normally, sprintf() takes the next unused argument as the value
               to format for each format specification. If the format
               specification uses "*" to require additional arguments, these
               are consumed from the argument list in the order they appear in
               the format specification before the value to format.  Where an
               argument is specified by an explicit index, this does not
               affect the normal order for the arguments, even when the
               explicitly specified index would have been the next argument.


                   printf "<%*.*s>", $a, $b, $c;

               uses $a for the width, $b for the precision, and $c as the
               value to format; while:

                 printf "<%*1$.*s>", $a, $b;

               would use $a for the width and precision, and $b as the value
               to format.

               Here are some more examples; be aware that when using an
               explicit index, the "$" may need escaping:

                 printf "%2\$d %d\n",    12, 34;        # will print "34 12\n"
                 printf "%2\$d %d %d\n", 12, 34;        # will print "34 12 34\n"
                 printf "%3\$d %d %d\n", 12, 34, 56;    # will print "56 12 34\n"
                 printf "%2\$*3\$d %d\n", 12, 34, 3;    # will print " 34 12\n"

           If "use locale" is in effect and POSIX::setlocale() has been
           called, the character used for the decimal separator in formatted
           floating-point numbers is affected by the LC_NUMERIC locale.  See
           perllocale and POSIX.

           The point of the function is to "seed" the "rand" function so that
           "rand" can produce a different sequence each time you run your
           program.  When called with a parameter, "srand" uses that for the
           seed; otherwise it (semi-)randomly chooses a seed.  In either case,
           starting with Perl 5.14, it returns the seed.

           If "srand()" is not called explicitly, it is called implicitly
           without a parameter at the first use of the "rand" operator.
           However, this was not true of versions of Perl before 5.004, so if
           your script will run under older Perl versions, it should call
           "srand"; otherwise most programs won't call "srand()" at all.

           But there are a few situations in recent Perls where programs are
           likely to want to call "srand".  One is for generating predictable
           results generally for testing or debugging.  There, you use
           "srand($seed)", with the same $seed each time.  Another other case
           is where you need a cryptographically-strong starting point rather
           than the generally acceptable default, which is based on time of
           day, process ID, and memory allocation, or the /dev/urandom device
           if available.  And still another case is that you may want to call
           "srand()" after a "fork()" to avoid child processes sharing the
           same seed value as the parent (and consequently each other).

           Do not call "srand()" (i.e., without an argument) more than once
           per process.  The internal state of the random number generator
           should contain more entropy than can be provided by any seed, so
           calling "srand()" again actually loses randomness.

           Most implementations of "srand" take an integer and will silently
           truncate decimal numbers.  This means "srand(42)" will usually
           produce the same results as "srand(42.1)".  To be safe, always pass
           "srand" an integer.

           In versions of Perl prior to 5.004 the default seed was just the
           current "time".  This isn't a particularly good seed, so many old
           programs supply their own seed value (often "time ^ $$" or "time ^
           ($$ + ($$ << 15))"), but that isn't necessary any more.

           For cryptographic purposes, however, you need something much more
           random than the default seed.  Checksumming the compressed output
           of one or more rapidly changing operating system status programs is
           the usual method.  For example:

               srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);

           If you're particularly concerned with this, search the CPAN for
           random number generator modules instead of rolling out your own.

           Frequently called programs (like CGI scripts) that simply use

               time ^ $$

           for a seed can fall prey to the mathematical property that

       stat EXPR
       stat DIRHANDLE
           Returns a 13-element list giving the status info for a file, either
           the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR.  If
           EXPR is omitted, it stats $_ (not "_"!).  Returns the empty list if
           "stat" fails.  Typically used as follows:

                      = stat($filename);

           Not all fields are supported on all filesystem types.  Here are the
           meanings of the fields:

             0 dev      device number of filesystem
             1 ino      inode number
             2 mode     file mode  (type and permissions)
             3 nlink    number of (hard) links to the file
             4 uid      numeric user ID of file's owner
             5 gid      numeric group ID of file's owner
             6 rdev     the device identifier (special files only)
             7 size     total size of file, in bytes
             8 atime    last access time in seconds since the epoch
             9 mtime    last modify time in seconds since the epoch
            10 ctime    inode change time in seconds since the epoch (*)
            11 blksize  preferred block size for file system I/O
            12 blocks   actual number of blocks allocated

           (The epoch was at 00:00 January 1, 1970 GMT.)

           (*) Not all fields are supported on all filesystem types. Notably,
           the ctime field is non-portable.  In particular, you cannot expect
           it to be a "creation time"; see "Files and Filesystems" in perlport
           for details.

           If "stat" is passed the special filehandle consisting of an
           underline, no stat is done, but the current contents of the stat
           structure from the last "stat", "lstat", or filetest are returned.

               if (-x $file && (($d) = stat(_)) && $d < 0) {
                   print "$file is executable NFS file\n";

           (This works on machines only for which the device number is
           negative under NFS.)

           Because the mode contains both the file type and its permissions,
           you should mask off the file type portion and (s)printf using a
           "%o" if you want to see the real permissions.

               $mode = (stat($filename))[2];
               printf "Permissions are %04o\n", $mode & 07777;
                      scalar localtime $sb->mtime;

           You can import symbolic mode constants ("S_IF*") and functions
           ("S_IS*") from the Fcntl module:

               use Fcntl ':mode';

               $mode = (stat($filename))[2];

               $user_rwx      = ($mode & S_IRWXU) >> 6;
               $group_read    = ($mode & S_IRGRP) >> 3;
               $other_execute =  $mode & S_IXOTH;

               printf "Permissions are %04o\n", S_IMODE($mode), "\n";

               $is_setuid     =  $mode & S_ISUID;
               $is_directory  =  S_ISDIR($mode);

           You could write the last two using the "-u" and "-d" operators.
           Commonly available "S_IF*" constants are:

               # Permissions: read, write, execute, for user, group, others.


               # Setuid/Setgid/Stickiness/SaveText.
               # Note that the exact meaning of these is system dependent.


               # File types.  Not necessarily all are available on your system.


               # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.

               S_IREAD S_IWRITE S_IEXEC

           and the "S_IF*" functions are

               S_IMODE($mode)    the part of $mode containing the permission bits
                       and the setuid/setgid/sticky bits

               S_IFMT($mode)    the part of $mode containing the file type
                       which can be bit-anded with (for example) S_IFREG
                                   or with the following functions

               # The operators -f, -d, -l, -b, -c, -p, and -S.

               S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
               S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)

       state TYPE EXPR
       state EXPR : ATTRS
       state TYPE EXPR : ATTRS
           "state" declares a lexically scoped variable, just like "my" does.
           However, those variables will never be reinitialized, contrary to
           lexical variables that are reinitialized each time their enclosing
           block is entered.

           "state" variables are enabled only when the "use feature "state""
           pragma is in effect.  See feature.

       study SCALAR
           Takes extra time to study SCALAR ($_ if unspecified) in
           anticipation of doing many pattern matches on the string before it
           is next modified.  This may or may not save time, depending on the
           nature and number of patterns you are searching and the
           distribution of character frequencies in the string to be searched;
           you probably want to compare run times with and without it to see
           which is faster.  Those loops that scan for many short constant
           strings (including the constant parts of more complex patterns)
           will benefit most.  You may have only one "study" active at a time:
           if you study a different scalar the first is "unstudied".  (The way
           "study" works is this: a linked list of every character in the
           string to be searched is made, so we know, for example, where all
           the 'k' characters are.  From each search string, the rarest
           character is selected, based on some static frequency tables
           constructed from some C programs and English text.  Only those
           places that contain this "rarest" character are examined.)

           For example, here is a loop that inserts index producing entries
           before any line containing a certain pattern:

               while (<>) {
                   print ".IX foo\n"    if /\bfoo\b/;
                   print ".IX bar\n"    if /\bbar\b/;
                   print ".IX blurfl\n" if /\bblurfl\b/;
                   # ...

           In searching for "/\bfoo\b/", only locations in $_ that contain "f"
           will be looked at, because "f" is rarer than "o".  In general, this
           is a big win except in pathological cases.  The only question is
           whether it saves you more time than it took to build the linked
           list in the first place.

           Note that if you have to look for strings that you don't know till
           runtime, you can build an entire loop as a string and "eval" that
           to avoid recompiling all your patterns all the time.  Together with
           undefining $/ to input entire files as one record, this can be
           quite fast, often faster than specialized programs like fgrep(1).
           The following scans a list of files (@files) for a list of words
               foreach $file (sort keys(%seen)) {
                   print $file, "\n";

       sub NAME BLOCK
       sub NAME (PROTO) BLOCK
       sub NAME : ATTRS BLOCK
           This is subroutine definition, not a real function per se.  Without
           a BLOCK it's just a forward declaration.  Without a NAME, it's an
           anonymous function declaration, so does return a value: the CODE
           ref of the closure just created.

           See perlsub and perlref for details about subroutines and
           references; see attributes and Attribute::Handlers for more
           information about attributes.

       substr EXPR,OFFSET,LENGTH
       substr EXPR,OFFSET
           Extracts a substring out of EXPR and returns it.  First character
           is at offset 0 (or whatever you've set $[ to (but <don't do that)).
           If OFFSET is negative (or more precisely, less than $[), starts
           that far back from the end of the string.  If LENGTH is omitted,
           returns everything through the end of the string.  If LENGTH is
           negative, leaves that many characters off the end of the string.

               my $s = "The black cat climbed the green tree";
               my $color  = substr $s, 4, 5;      # black
               my $middle = substr $s, 4, -11;    # black cat climbed the
               my $end    = substr $s, 14;        # climbed the green tree
               my $tail   = substr $s, -4;        # tree
               my $z      = substr $s, -4, 2;     # tr

           You can use the substr() function as an lvalue, in which case EXPR
           must itself be an lvalue.  If you assign something shorter than
           LENGTH, the string will shrink, and if you assign something longer
           than LENGTH, the string will grow to accommodate it.  To keep the
           string the same length, you may need to pad or chop your value
           using "sprintf".

           If OFFSET and LENGTH specify a substring that is partly outside the
           string, only the part within the string is returned.  If the
           substring is beyond either end of the string, substr() returns the
           undefined value and produces a warning.  When used as an lvalue,
           specifying a substring that is entirely outside the string raises
           an exception.  Here's an example showing the behavior for boundary

               my $name = 'fred';
               substr($name, 4) = 'dy';         # $name is now 'freddy'
               my $null = substr $name, 6, 2;   # returns "" (no warning)
               my $oops = substr $name, 7;      # returns undef, with warning
               substr($name, 7) = 'gap';        # raises an exception

           remembers which part of the original string is being modified; for

               $x = '1234';
               for (substr($x,1,2)) {
                   $_ = 'a';   print $x,"\n";    # prints 1a4
                   $_ = 'xyz'; print $x,"\n";    # prints 1xyz4
                   $x = '56789';
                   $_ = 'pq';  print $x,"\n";    # prints 5pq9

           Prior to Perl version 5.9.1, the result of using an lvalue multiple
           times was unspecified.

       symlink OLDFILE,NEWFILE
           Creates a new filename symbolically linked to the old filename.
           Returns 1 for success, 0 otherwise.  On systems that don't support
           symbolic links, raises an exception.  To check for that, use eval:

               $symlink_exists = eval { symlink("",""); 1 };

       syscall NUMBER, LIST
           Calls the system call specified as the first element of the list,
           passing the remaining elements as arguments to the system call.  If
           unimplemented, raises an exception.  The arguments are interpreted
           as follows: if a given argument is numeric, the argument is passed
           as an int.  If not, the pointer to the string value is passed.  You
           are responsible to make sure a string is pre-extended long enough
           to receive any result that might be written into a string.  You
           can't use a string literal (or other read-only string) as an
           argument to "syscall" because Perl has to assume that any string
           pointer might be written through.  If your integer arguments are
           not literals and have never been interpreted in a numeric context,
           you may need to add 0 to them to force them to look like numbers.
           This emulates the "syswrite" function (or vice versa):

               require '';        # may need to run h2ph
               $s = "hi there\n";
               syscall(&SYS_write, fileno(STDOUT), $s, length $s);

           Note that Perl supports passing of up to only 14 arguments to your
           syscall, which in practice should (usually) suffice.

           Syscall returns whatever value returned by the system call it
           calls.  If the system call fails, "syscall" returns "-1" and sets
           $! (errno).  Note that some system calls can legitimately return
           "-1".  The proper way to handle such calls is to assign "$!=0"
           before the call, then check the value of $! if "syscall" returns

           There's a problem with "syscall(&SYS_pipe)": it returns the file
           number of the read end of the pipe it creates, but there is no way
           to retrieve the file number of the other end.  You can avoid this
           problem by using "pipe" instead.
           the documentation of your operating system's open(2) syscall to see
           which values and flag bits are available.  You may combine several
           flags using the "|"-operator.

           Some of the most common values are "O_RDONLY" for opening the file
           in read-only mode, "O_WRONLY" for opening the file in write-only
           mode, and "O_RDWR" for opening the file in read-write mode.

           For historical reasons, some values work on almost every system
           supported by Perl: 0 means read-only, 1 means write-only, and 2
           means read/write.  We know that these values do not work under
           OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want
           to use them in new code.

           If the file named by FILENAME does not exist and the "open" call
           creates it (typically because MODE includes the "O_CREAT" flag),
           then the value of PERMS specifies the permissions of the newly
           created file.  If you omit the PERMS argument to "sysopen", Perl
           uses the octal value 0666.  These permission values need to be in
           octal, and are modified by your process's current "umask".

           In many systems the "O_EXCL" flag is available for opening files in
           exclusive mode.  This is not locking: exclusiveness means here that
           if the file already exists, sysopen() fails.  "O_EXCL" may not work
           on network filesystems, and has no effect unless the "O_CREAT" flag
           is set as well.  Setting "O_CREAT|O_EXCL" prevents the file from
           being opened if it is a symbolic link.  It does not protect against
           symbolic links in the file's path.

           Sometimes you may want to truncate an already-existing file.  This
           can be done using the "O_TRUNC" flag.  The behavior of "O_TRUNC"
           with "O_RDONLY" is undefined.

           You should seldom if ever use 0644 as argument to "sysopen",
           because that takes away the user's option to have a more permissive
           umask.  Better to omit it.  See the perlfunc(1) entry on "umask"
           for more on this.

           Note that "sysopen" depends on the fdopen() C library function.  On
           many Unix systems, fdopen() is known to fail when file descriptors
           exceed a certain value, typically 255. If you need more file
           descriptors than that, consider rebuilding Perl to use the "sfio"
           library, or perhaps using the POSIX::open() function.

           See perlopentut for a kinder, gentler explanation of opening files.

           Attempts to read LENGTH bytes of data into variable SCALAR from the
           specified FILEHANDLE, using the read(2).  It bypasses buffered IO,
           so mixing this with other kinds of reads, "print", "write", "seek",
           "tell", or "eof" can cause confusion because the perlio or stdio
           layers usually buffers data.  Returns the number of bytes actually
           read, 0 at end of file, or undef if there was an error (in the
           work well on device files (like ttys) anyway.  Use sysread() and
           check for a return value for 0 to decide whether you're done.

           Note that if the filehandle has been marked as ":utf8" Unicode
           characters are read instead of bytes (the LENGTH, OFFSET, and the
           return value of sysread() are in Unicode characters).  The
           ":encoding(...)" layer implicitly introduces the ":utf8" layer.
           See "binmode", "open", and the "open" pragma, open.

           Sets FILEHANDLE's system position in bytes using lseek(2).
           FILEHANDLE may be an expression whose value gives the name of the
           filehandle.  The values for WHENCE are 0 to set the new position to
           POSITION; 1 to set the it to the current position plus POSITION;
           and 2 to set it to EOF plus POSITION, typically negative.

           Note the in bytes: even if the filehandle has been set to operate
           on characters (for example by using the ":encoding(utf8)" I/O
           layer), tell() will return byte offsets, not character offsets
           (because implementing that would render sysseek() unacceptably

           sysseek() bypasses normal buffered IO, so mixing it with reads
           other than "sysread" (for example "<>" or read()) "print", "write",
           "seek", "tell", or "eof" may cause confusion.

           For WHENCE, you may also use the constants "SEEK_SET", "SEEK_CUR",
           and "SEEK_END" (start of the file, current position, end of the
           file) from the Fcntl module.  Use of the constants is also more
           portable than relying on 0, 1, and 2.  For example to define a
           "systell" function:

               use Fcntl 'SEEK_CUR';
               sub systell { sysseek($_[0], 0, SEEK_CUR) }

           Returns the new position, or the undefined value on failure.  A
           position of zero is returned as the string "0 but true"; thus
           "sysseek" returns true on success and false on failure, yet you can
           still easily determine the new position.

       system LIST
       system PROGRAM LIST
           Does exactly the same thing as "exec LIST", except that a fork is
           done first and the parent process waits for the child process to
           exit.  Note that argument processing varies depending on the number
           of arguments.  If there is more than one argument in LIST, or if
           LIST is an array with more than one value, starts the program given
           by the first element of the list with arguments given by the rest
           of the list.  If there is only one scalar argument, the argument is
           checked for shell metacharacters, and if there are any, the entire
           argument is passed to the system's command shell for parsing (this
           is "/bin/sh -c" on Unix platforms, but varies on other platforms).
           If there are no shell metacharacters in the argument, it is split
           into words and passed directly to "execvp", which is more
           merely backticks or "qx//", as described in "`STRING`" in perlop.
           Return value of -1 indicates a failure to start the program or an
           error of the wait(2) system call (inspect $! for the reason).

           If you'd like to make "system" (and many other bits of Perl) die on
           error, have a look at the autodie pragma.

           Like "exec", "system" allows you to lie to a program about its name
           if you use the "system PROGRAM LIST" syntax.  Again, see "exec".

           Since "SIGINT" and "SIGQUIT" are ignored during the execution of
           "system", if you expect your program to terminate on receipt of
           these signals you will need to arrange to do so yourself based on
           the return value.

               @args = ("command", "arg1", "arg2");
               system(@args) == 0
                   or die "system @args failed: $?"

           If you'd like to manually inspect "system"'s failure, you can check
           all possible failure modes by inspecting $? like this:

               if ($? == -1) {
                   print "failed to execute: $!\n";
               elsif ($? & 127) {
                   printf "child died with signal %d, %s coredump\n",
                       ($? & 127),  ($? & 128) ? 'with' : 'without';
               else {
                   printf "child exited with value %d\n", $? >> 8;

           Alternatively, you may inspect the value of
           "${^CHILD_ERROR_NATIVE}" with the "W*()" calls from the POSIX

           When "system"'s arguments are executed indirectly by the shell,
           results and return codes are subject to its quirks.  See "`STRING`"
           in perlop and "exec" for details.

           Since "system" does a "fork" and "wait" it may affect a "SIGCHLD"
           handler. See perlipc for details.

       syswrite FILEHANDLE,SCALAR
           Attempts to write LENGTH bytes of data from variable SCALAR to the
           specified FILEHANDLE, using write(2).  If LENGTH is not specified,
           writes whole SCALAR.  It bypasses buffered IO, so mixing this with
           reads (other than sysread()), "print", "write", "seek", "tell", or
           "eof" may cause confusion because the perlio and stdio layers
           usually buffer data.  Returns the number of bytes actually written,
           or "undef" if there was an error (in this case the errno variable
           OFFSET, and return value of syswrite() are in (UTF8-encoded
           Unicode) characters.  The ":encoding(...)" layer implicitly
           introduces the ":utf8" layer.  Alternately, if the handle is not
           marked with an encoding but you attempt to write characters with
           code points over 255, raises an exception.  See "binmode", "open",
           and the "open" pragma, open.

       tell FILEHANDLE
           Returns the current position in bytes for FILEHANDLE, or -1 on
           error.  FILEHANDLE may be an expression whose value gives the name
           of the actual filehandle.  If FILEHANDLE is omitted, assumes the
           file last read.

           Note the in bytes: even if the filehandle has been set to operate
           on characters (for example by using the ":encoding(utf8)" open
           layer), tell() will return byte offsets, not character offsets
           (because that would render seek() and tell() rather slow).

           The return value of tell() for the standard streams like the STDIN
           depends on the operating system: it may return -1 or something
           else.  tell() on pipes, fifos, and sockets usually returns -1.

           There is no "systell" function.  Use "sysseek(FH, 0, 1)" for that.

           Do not use tell() (or other buffered I/O operations) on a
           filehandle that has been manipulated by sysread(), syswrite(), or
           sysseek().  Those functions ignore the buffering, while tell() does

       telldir DIRHANDLE
           Returns the current position of the "readdir" routines on
           DIRHANDLE.  Value may be given to "seekdir" to access a particular
           location in a directory.  "telldir" has the same caveats about
           possible directory compaction as the corresponding system library

           This function binds a variable to a package class that will provide
           the implementation for the variable.  VARIABLE is the name of the
           variable to be enchanted.  CLASSNAME is the name of a class
           implementing objects of correct type.  Any additional arguments are
           passed to the "new" method of the class (meaning "TIESCALAR",
           "TIEHANDLE", "TIEARRAY", or "TIEHASH").  Typically these are
           arguments such as might be passed to the "dbm_open()" function of
           C.  The object returned by the "new" method is also returned by the
           "tie" function, which would be useful if you want to access other
           methods in CLASSNAME.

           Note that functions such as "keys" and "values" may return huge
           lists when used on large objects, like DBM files.  You may prefer
           to use the "each" function to iterate over such.  Example:

               # print out history file offsets
               DELETE this, key
               CLEAR this
               EXISTS this, key
               FIRSTKEY this
               NEXTKEY this, lastkey
               SCALAR this
               DESTROY this
               UNTIE this

           A class implementing an ordinary array should have the following

               TIEARRAY classname, LIST
               FETCH this, key
               STORE this, key, value
               FETCHSIZE this
               STORESIZE this, count
               CLEAR this
               PUSH this, LIST
               POP this
               SHIFT this
               UNSHIFT this, LIST
               SPLICE this, offset, length, LIST
               EXTEND this, count
               DESTROY this
               UNTIE this

           A class implementing a filehandle should have the following

               TIEHANDLE classname, LIST
               READ this, scalar, length, offset
               READLINE this
               GETC this
               WRITE this, scalar, length, offset
               PRINT this, LIST
               PRINTF this, format, LIST
               BINMODE this
               EOF this
               FILENO this
               SEEK this, position, whence
               TELL this
               OPEN this, mode, LIST
               CLOSE this
               DESTROY this
               UNTIE this

           A class implementing a scalar should have the following methods:

               TIESCALAR classname, LIST
               FETCH this,
               STORE this, value
               DESTROY this
               UNTIE this

           value that was originally returned by the "tie" call that bound the
           variable to a package.)  Returns the undefined value if VARIABLE
           isn't tied to a package.

           Returns the number of non-leap seconds since whatever time the
           system considers to be the epoch, suitable for feeding to "gmtime"
           and "localtime". On most systems the epoch is 00:00:00 UTC, January
           1, 1970; a prominent exception being Mac OS Classic which uses
           00:00:00, January 1, 1904 in the current local time zone for its

           For measuring time in better granularity than one second, use the
           Time::HiRes module from Perl 5.8 onwards (or from CPAN before
           then), or, if you have gettimeofday(2), you may be able to use the
           "syscall" interface of Perl.  See perlfaq8 for details.

           For date and time processing look at the many related modules on
           CPAN.  For a comprehensive date and time representation look at the
           DateTime module.

           Returns a four-element list giving the user and system times in
           seconds for this process and any exited children of this process.

               ($user,$system,$cuser,$csystem) = times;

           In scalar context, "times" returns $user.

           Children's times are only included for terminated children.

           The transliteration operator.  Same as "y///".  See "Quote and
           Quote-like Operators" in perlop.

       truncate FILEHANDLE,LENGTH
       truncate EXPR,LENGTH
           Truncates the file opened on FILEHANDLE, or named by EXPR, to the
           specified length.  Raises an exception if truncate isn't
           implemented on your system.  Returns true if successful, "undef" on

           The behavior is undefined if LENGTH is greater than the length of
           the file.

           The position in the file of FILEHANDLE is left unchanged.  You may
           want to call seek before writing to the file.

       uc EXPR
       uc  Returns an uppercased version of EXPR.  This is the internal
           function implementing the "\U" escape in double-quoted strings.  It
           does not attempt to do titlecase mapping on initial letters.  See
           "ucfirst" for that.

           This function behaves the same way under various pragma, such as in
           a locale, as "lc" does.

       umask EXPR
           Sets the umask for the process to EXPR and returns the previous
           value.  If EXPR is omitted, merely returns the current umask.

           The Unix permission "rwxr-x---" is represented as three sets of
           three bits, or three octal digits: 0750 (the leading 0 indicates
           octal and isn't one of the digits).  The "umask" value is such a
           number representing disabled permissions bits.  The permission (or
           "mode") values you pass "mkdir" or "sysopen" are modified by your
           umask, so even if you tell "sysopen" to create a file with
           permissions 0777, if your umask is 0022, then the file will
           actually be created with permissions 0755.  If your "umask" were
           0027 (group can't write; others can't read, write, or execute),
           then passing "sysopen" 0666 would create a file with mode 0640
           (because "0666 &~ 027" is 0640).

           Here's some advice: supply a creation mode of 0666 for regular
           files (in "sysopen") and one of 0777 for directories (in "mkdir")
           and executable files.  This gives users the freedom of choice: if
           they want protected files, they might choose process umasks of 022,
           027, or even the particularly antisocial mask of 077.  Programs
           should rarely if ever make policy decisions better left to the
           user.  The exception to this is when writing files that should be
           kept private: mail files, web browser cookies, .rhosts files, and
           so on.

           If umask(2) is not implemented on your system and you are trying to
           restrict access for yourself (i.e., "(EXPR & 0700) > 0"), raises an
           exception.  If umask(2) is not implemented and you are not trying
           to restrict access for yourself, returns "undef".

           Remember that a umask is a number, usually given in octal; it is
           not a string of octal digits.  See also "oct", if all you have is a

       undef EXPR
           Undefines the value of EXPR, which must be an lvalue.  Use only on
           a scalar value, an array (using "@"), a hash (using "%"), a
           subroutine (using "&"), or a typeglob (using "*").  Saying "undef
           $hash{$key}" will probably not do what you expect on most
           predefined variables or DBM list values, so don't do that; see
           delete.  Always returns the undefined value.  You can omit the
           EXPR, in which case nothing is undefined, but you still get an
           undefined value that you could, for instance, return from a
           subroutine, assign to a variable, or pass as a parameter.

               undef $foo;

           Deletes a list of files. On success, it returns the number of files
           it successfully deleted. On failure, it returns false and sets $!

               my $unlinked = unlink 'a', 'b', 'c';
               unlink @goners;
               unlink glob "*.bak";

           On error, "unlink" will not tell you which files it could not
           remove.  If you want to know which files you could not remove, try
           them one at a time:

                foreach my $file ( @goners ) {
                    unlink $file or warn "Could not unlink $file: $!";

           Note: "unlink" will not attempt to delete directories unless you
           are superuser and the -U flag is supplied to Perl. Even if these
           conditions are met, be warned that unlinking a directory can
           inflict damage on your filesystem.  Finally, using "unlink" on
           directories is not supported on many operating systems.  Use
           "rmdir" instead.

           If LIST is omitted, "unlink" uses $_.

       unpack TEMPLATE,EXPR
       unpack TEMPLATE
           "unpack" does the reverse of "pack": it takes a string and expands
           it out into a list of values.  (In scalar context, it returns
           merely the first value produced.)

           If EXPR is omitted, unpacks the $_ string.  See perlpacktut for an
           introduction to this function.

           The string is broken into chunks described by the TEMPLATE.  Each
           chunk is converted separately to a value.  Typically, either the
           string is a result of "pack", or the characters of the string
           represent a C structure of some kind.

           The TEMPLATE has the same format as in the "pack" function.  Here's
           a subroutine that does substring:

               sub substr {
                   my($what,$where,$howmuch) = @_;
                   unpack("x$where a$howmuch", $what);

           and then there's

               sub ordinal { unpack("W",$_[0]); } # same as ord()

           In addition to fields allowed in pack(), you may prefix a field
           with a %<number> to indicate that you want a <number>-bit checksum

           The following efficiently counts the number of set bits in a bit

               $setbits = unpack("%32b*", $selectmask);

           The "p" and "P" formats should be used with care.  Since Perl has
           no way of checking whether the value passed to "unpack()"
           corresponds to a valid memory location, passing a pointer value
           that's not known to be valid is likely to have disastrous

           If there are more pack codes or if the repeat count of a field or a
           group is larger than what the remainder of the input string allows,
           the result is not well defined: the repeat count may be decreased,
           or "unpack()" may produce empty strings or zeros, or it may raise
           an exception.  If the input string is longer than one described by
           the TEMPLATE, the remainder of that input string is ignored.

           See "pack" for more examples and notes.

       untie VARIABLE
           Breaks the binding between a variable and a package.  (See "tie".)
           Has no effect if the variable is not tied.

       unshift ARRAY,LIST
       unshift EXPR,LIST
           Does the opposite of a "shift".  Or the opposite of a "push",
           depending on how you look at it.  Prepends list to the front of the
           array and returns the new number of elements in the array.

               unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;

           Note the LIST is prepended whole, not one element at a time, so the
           prepended elements stay in the same order.  Use "reverse" to do the

           Starting with Perl 5.14, "unshift" can take a scalar EXPR, which
           must hold a reference to an unblessed array.  The argument will be
           dereferenced automatically.  This aspect of "unshift" is considered
           highly experimental.  The exact behaviour may change in a future
           version of Perl.

       use Module VERSION LIST
       use Module VERSION
       use Module LIST
       use Module
       use VERSION
           Imports some semantics into the current package from the named
           module, generally by aliasing certain subroutine or variable names
           into your package.  It is exactly equivalent to

               BEGIN { require Module; Module->import( LIST ); }

           Specifying VERSION as a literal of the form v5.6.1 should generally
           be avoided, because it leads to misleading error messages under
           earlier versions of Perl (that is, prior to 5.6.0) that do not
           support this syntax.  The equivalent numeric version should be used

               use v5.6.1;     # compile time version check
               use 5.6.1;      # ditto
               use 5.006_001;  # ditto; preferred for backwards compatibility

           This is often useful if you need to check the current Perl version
           before "use"ing library modules that won't work with older versions
           of Perl.  (We try not to do this more than we have to.)

           Also, if the specified Perl version is greater than or equal to
           5.9.5, "use VERSION" will also load the "feature" pragma and enable
           all features available in the requested version.  See feature.
           Similarly, if the specified Perl version is greater than or equal
           to 5.11.0, strictures are enabled lexically as with "use strict"
           (except that the file is not actually loaded).

           The "BEGIN" forces the "require" and "import" to happen at compile
           time.  The "require" makes sure the module is loaded into memory if
           it hasn't been yet.  The "import" is not a builtin; it's just an
           ordinary static method call into the "Module" package to tell the
           module to import the list of features back into the current
           package.  The module can implement its "import" method any way it
           likes, though most modules just choose to derive their "import"
           method via inheritance from the "Exporter" class that is defined in
           the "Exporter" module.  See Exporter.  If no "import" method can be
           found then the call is skipped, even if there is an AUTOLOAD

           If you do not want to call the package's "import" method (for
           instance, to stop your namespace from being altered), explicitly
           supply the empty list:

               use Module ();

           That is exactly equivalent to

               BEGIN { require Module }

           If the VERSION argument is present between Module and LIST, then
           the "use" will call the VERSION method in class Module with the
           given version as an argument.  The default VERSION method,
           inherited from the UNIVERSAL class, croaks if the given version is
           larger than the value of the variable $Module::VERSION.

           Again, there is a distinction between omitting LIST ("import"
           called with no arguments) and an explicit empty LIST "()" ("import"
           not called).  Note that there is no comma after VERSION!

           Some of these pseudo-modules import semantics into the current
           block scope (like "strict" or "integer", unlike ordinary modules,
           which import symbols into the current package (which are effective
           through the end of the file).

           Because "use" takes effect at compile time, it doesn't respect the
           ordinary flow control of the code being compiled.  In particular,
           putting a "use" inside the false branch of a conditional doesn't
           prevent it from being processed.  If a module or pragma only needs
           to be loaded conditionally, this can be done using the if pragma:

               use if $] < 5.008, "utf8";
               use if WANT_WARNINGS, warnings => qw(all);

           There's a corresponding "no" declaration that unimports meanings
           imported by "use", i.e., it calls "unimport Module LIST" instead of
           "import".  It behaves just as "import" does with VERSION, an
           omitted or empty LIST, or no unimport method being found.

               no integer;
               no strict 'refs';
               no warnings;

           Care should be taken when using the "no VERSION" form of "no".  It
           is only meant to be used to assert that the running Perl is of a
           earlier version than its argument and not to undo the feature-
           enabling side effects of "use VERSION".

           See perlmodlib for a list of standard modules and pragmas.  See
           perlrun for the "-M" and "-m" command-line options to Perl that
           give "use" functionality from the command-line.

       utime LIST
           Changes the access and modification times on each file of a list of
           files.  The first two elements of the list must be the NUMERIC
           access and modification times, in that order.  Returns the number
           of files successfully changed.  The inode change time of each file
           is set to the current time.  For example, this code has the same
           effect as the Unix touch(1) command when the files already exist
           and belong to the user running the program:

               $atime = $mtime = time;
               utime $atime, $mtime, @ARGV;

           Since Perl 5.7.2, if the first two elements of the list are
           "undef", the utime(2) syscall from your C library is called with a
           null second argument. On most systems, this will set the file's
           access and modification times to the current time (i.e., equivalent
           to the example above) and will work even on files you don't own
           provided you have write permission:

               for $file (@ARGV) {

           "undef".  This also triggers an uninitialized warning.

           On systems that support futimes(2), you may pass filehandles among
           the files.  On systems that don't support futimes(2), passing
           filehandles raises an exception.  Filehandles must be passed as
           globs or glob references to be recognized; barewords are considered

       values HASH
       values ARRAY
       values EXPR
           Returns a list consisting of all the values of the named hash, or
           the values of an array. (In scalar context, returns the number of

           The values are returned in an apparently random order.  The actual
           random order is subject to change in future versions of Perl, but
           it is guaranteed to be the same order as either the "keys" or
           "each" function would produce on the same (unmodified) hash.  Since
           Perl 5.8.1 the ordering is different even between different runs of
           Perl for security reasons (see "Algorithmic Complexity Attacks" in

           As a side effect, calling values() resets the HASH or ARRAY's
           internal iterator; see "each". (In particular, calling values() in
           void context resets the iterator with no other overhead. Apart from
           resetting the iterator, "values @array" in list context is the same
           as plain @array.  We recommend that you use void context "keys
           @array" for this, but reasoned that it taking "values @array" out
           would require more documentation than leaving it in.)

           Note that the values are not copied, which means modifying them
           will modify the contents of the hash:

               for (values %hash)      { s/foo/bar/g }   # modifies %hash values
               for (@hash{keys %hash}) { s/foo/bar/g }   # same

           Starting with Perl 5.14, "values" can take a scalar EXPR, which
           must hold a reference to an unblessed hash or array.  The argument
           will be dereferenced automatically.  This aspect of "values" is
           considered highly experimental.  The exact behaviour may change in
           a future version of Perl.

               for (values $hashref) { ... }
               for (values $obj->get_arrayref) { ... }

           See also "keys", "each", and "sort".

           Treats the string in EXPR as a bit vector made up of elements of
           width BITS and returns the value of the element specified by OFFSET
           as an unsigned integer.  BITS therefore specifies the number of
           bits that are reserved for each element in the bit vector.  This
           must be a power of two from 1 to 32 (or 64, if your platform
           0x08, 0x10, 0x20, 0x40, 0x80.  For example, breaking the single
           input byte "chr(0x36)" into two groups gives a list "(0x6, 0x3)";
           breaking it into 4 groups gives "(0x2, 0x1, 0x3, 0x0)".

           "vec" may also be assigned to, in which case parentheses are needed
           to give the expression the correct precedence as in

               vec($image, $max_x * $x + $y, 8) = 3;

           If the selected element is outside the string, the value 0 is
           returned.  If an element off the end of the string is written to,
           Perl will first extend the string with sufficiently many zero
           bytes.   It is an error to try to write off the beginning of the
           string (i.e., negative OFFSET).

           If the string happens to be encoded as UTF-8 internally (and thus
           has the UTF8 flag set), this is ignored by "vec", and it operates
           on the internal byte string, not the conceptual character string,
           even if you only have characters with values less than 256.

           Strings created with "vec" can also be manipulated with the logical
           operators "|", "&", "^", and "~".  These operators will assume a
           bit vector operation is desired when both operands are strings.
           See "Bitwise String Operators" in perlop.

           The following code will build up an ASCII string saying
           'PerlPerlPerl'.  The comments show the string after each step.
           Note that this code works in the same way on big-endian or little-
           endian machines.

               my $foo = '';
               vec($foo,  0, 32) = 0x5065726C; # 'Perl'

               # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
               print vec($foo, 0, 8);  # prints 80 == 0x50 == ord('P')

               vec($foo,  2, 16) = 0x5065; # 'PerlPe'
               vec($foo,  3, 16) = 0x726C; # 'PerlPerl'
               vec($foo,  8,  8) = 0x50;   # 'PerlPerlP'
               vec($foo,  9,  8) = 0x65;   # 'PerlPerlPe'
               vec($foo, 20,  4) = 2;      # 'PerlPerlPe'   . "\x02"
               vec($foo, 21,  4) = 7;      # 'PerlPerlPer'
                                              # 'r' is "\x72"
               vec($foo, 45,  2) = 3;      # 'PerlPerlPer'  . "\x0c"
               vec($foo, 93,  1) = 1;      # 'PerlPerlPer'  . "\x2c"
               vec($foo, 94,  1) = 1;      # 'PerlPerlPerl'
                                              # 'l' is "\x6c"

           To transform a bit vector into a string or list of 0's and 1's, use

               $bits = unpack("b*", $vector);
               @bits = split(//, unpack("b*", $vector));


               for $w (0..3) {
                   $width = 2**$w;
                   for ($shift=0; $shift < $width; ++$shift) {
                       for ($off=0; $off < 32/$width; ++$off) {
                           $str = pack("B*", "0"x32);
                           $bits = (1<<$shift);
                           vec($str, $off, $width) = $bits;
                           $res = unpack("b*",$str);
                           $val = unpack("V", $str);

               format STDOUT =
               vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
               $off, $width, $bits, $val, $res

           Regardless of the machine architecture on which it runs, the
           example above should print the following table:

                                                 0         1         2         3
                                  unpack("V",$_) 01234567890123456789012345678901
               vec($_, 0, 1) = 1   ==          1 10000000000000000000000000000000
               vec($_, 1, 1) = 1   ==          2 01000000000000000000000000000000
               vec($_, 2, 1) = 1   ==          4 00100000000000000000000000000000
               vec($_, 3, 1) = 1   ==          8 00010000000000000000000000000000
               vec($_, 4, 1) = 1   ==         16 00001000000000000000000000000000
               vec($_, 5, 1) = 1   ==         32 00000100000000000000000000000000
               vec($_, 6, 1) = 1   ==         64 00000010000000000000000000000000
               vec($_, 7, 1) = 1   ==        128 00000001000000000000000000000000
               vec($_, 8, 1) = 1   ==        256 00000000100000000000000000000000
               vec($_, 9, 1) = 1   ==        512 00000000010000000000000000000000
               vec($_,10, 1) = 1   ==       1024 00000000001000000000000000000000
               vec($_,11, 1) = 1   ==       2048 00000000000100000000000000000000
               vec($_,12, 1) = 1   ==       4096 00000000000010000000000000000000
               vec($_,13, 1) = 1   ==       8192 00000000000001000000000000000000
               vec($_,14, 1) = 1   ==      16384 00000000000000100000000000000000
               vec($_,15, 1) = 1   ==      32768 00000000000000010000000000000000
               vec($_,16, 1) = 1   ==      65536 00000000000000001000000000000000
               vec($_,17, 1) = 1   ==     131072 00000000000000000100000000000000
               vec($_,18, 1) = 1   ==     262144 00000000000000000010000000000000
               vec($_,19, 1) = 1   ==     524288 00000000000000000001000000000000
               vec($_,20, 1) = 1   ==    1048576 00000000000000000000100000000000
               vec($_,21, 1) = 1   ==    2097152 00000000000000000000010000000000
               vec($_,22, 1) = 1   ==    4194304 00000000000000000000001000000000
               vec($_,23, 1) = 1   ==    8388608 00000000000000000000000100000000
               vec($_,24, 1) = 1   ==   16777216 00000000000000000000000010000000
               vec($_,25, 1) = 1   ==   33554432 00000000000000000000000001000000
               vec($_, 6, 2) = 1   ==       4096 00000000000010000000000000000000
               vec($_, 7, 2) = 1   ==      16384 00000000000000100000000000000000
               vec($_, 8, 2) = 1   ==      65536 00000000000000001000000000000000
               vec($_, 9, 2) = 1   ==     262144 00000000000000000010000000000000
               vec($_,10, 2) = 1   ==    1048576 00000000000000000000100000000000
               vec($_,11, 2) = 1   ==    4194304 00000000000000000000001000000000
               vec($_,12, 2) = 1   ==   16777216 00000000000000000000000010000000
               vec($_,13, 2) = 1   ==   67108864 00000000000000000000000000100000
               vec($_,14, 2) = 1   ==  268435456 00000000000000000000000000001000
               vec($_,15, 2) = 1   == 1073741824 00000000000000000000000000000010
               vec($_, 0, 2) = 2   ==          2 01000000000000000000000000000000
               vec($_, 1, 2) = 2   ==          8 00010000000000000000000000000000
               vec($_, 2, 2) = 2   ==         32 00000100000000000000000000000000
               vec($_, 3, 2) = 2   ==        128 00000001000000000000000000000000
               vec($_, 4, 2) = 2   ==        512 00000000010000000000000000000000
               vec($_, 5, 2) = 2   ==       2048 00000000000100000000000000000000
               vec($_, 6, 2) = 2   ==       8192 00000000000001000000000000000000
               vec($_, 7, 2) = 2   ==      32768 00000000000000010000000000000000
               vec($_, 8, 2) = 2   ==     131072 00000000000000000100000000000000
               vec($_, 9, 2) = 2   ==     524288 00000000000000000001000000000000
               vec($_,10, 2) = 2   ==    2097152 00000000000000000000010000000000
               vec($_,11, 2) = 2   ==    8388608 00000000000000000000000100000000
               vec($_,12, 2) = 2   ==   33554432 00000000000000000000000001000000
               vec($_,13, 2) = 2   ==  134217728 00000000000000000000000000010000
               vec($_,14, 2) = 2   ==  536870912 00000000000000000000000000000100
               vec($_,15, 2) = 2   == 2147483648 00000000000000000000000000000001
               vec($_, 0, 4) = 1   ==          1 10000000000000000000000000000000
               vec($_, 1, 4) = 1   ==         16 00001000000000000000000000000000
               vec($_, 2, 4) = 1   ==        256 00000000100000000000000000000000
               vec($_, 3, 4) = 1   ==       4096 00000000000010000000000000000000
               vec($_, 4, 4) = 1   ==      65536 00000000000000001000000000000000
               vec($_, 5, 4) = 1   ==    1048576 00000000000000000000100000000000
               vec($_, 6, 4) = 1   ==   16777216 00000000000000000000000010000000
               vec($_, 7, 4) = 1   ==  268435456 00000000000000000000000000001000
               vec($_, 0, 4) = 2   ==          2 01000000000000000000000000000000
               vec($_, 1, 4) = 2   ==         32 00000100000000000000000000000000
               vec($_, 2, 4) = 2   ==        512 00000000010000000000000000000000
               vec($_, 3, 4) = 2   ==       8192 00000000000001000000000000000000
               vec($_, 4, 4) = 2   ==     131072 00000000000000000100000000000000
               vec($_, 5, 4) = 2   ==    2097152 00000000000000000000010000000000
               vec($_, 6, 4) = 2   ==   33554432 00000000000000000000000001000000
               vec($_, 7, 4) = 2   ==  536870912 00000000000000000000000000000100
               vec($_, 0, 4) = 4   ==          4 00100000000000000000000000000000
               vec($_, 1, 4) = 4   ==         64 00000010000000000000000000000000
               vec($_, 2, 4) = 4   ==       1024 00000000001000000000000000000000
               vec($_, 3, 4) = 4   ==      16384 00000000000000100000000000000000
               vec($_, 4, 4) = 4   ==     262144 00000000000000000010000000000000
               vec($_, 5, 4) = 4   ==    4194304 00000000000000000000001000000000
               vec($_, 6, 4) = 4   ==   67108864 00000000000000000000000000100000
               vec($_, 7, 4) = 4   == 1073741824 00000000000000000000000000000010
               vec($_, 0, 4) = 8   ==          8 00010000000000000000000000000000
               vec($_, 1, 4) = 8   ==        128 00000001000000000000000000000000
               vec($_, 2, 4) = 8   ==       2048 00000000000100000000000000000000
               vec($_, 3, 4) = 8   ==      32768 00000000000000010000000000000000
               vec($_, 0, 8) = 4   ==          4 00100000000000000000000000000000
               vec($_, 1, 8) = 4   ==       1024 00000000001000000000000000000000
               vec($_, 2, 8) = 4   ==     262144 00000000000000000010000000000000
               vec($_, 3, 8) = 4   ==   67108864 00000000000000000000000000100000
               vec($_, 0, 8) = 8   ==          8 00010000000000000000000000000000
               vec($_, 1, 8) = 8   ==       2048 00000000000100000000000000000000
               vec($_, 2, 8) = 8   ==     524288 00000000000000000001000000000000
               vec($_, 3, 8) = 8   ==  134217728 00000000000000000000000000010000
               vec($_, 0, 8) = 16  ==         16 00001000000000000000000000000000
               vec($_, 1, 8) = 16  ==       4096 00000000000010000000000000000000
               vec($_, 2, 8) = 16  ==    1048576 00000000000000000000100000000000
               vec($_, 3, 8) = 16  ==  268435456 00000000000000000000000000001000
               vec($_, 0, 8) = 32  ==         32 00000100000000000000000000000000
               vec($_, 1, 8) = 32  ==       8192 00000000000001000000000000000000
               vec($_, 2, 8) = 32  ==    2097152 00000000000000000000010000000000
               vec($_, 3, 8) = 32  ==  536870912 00000000000000000000000000000100
               vec($_, 0, 8) = 64  ==         64 00000010000000000000000000000000
               vec($_, 1, 8) = 64  ==      16384 00000000000000100000000000000000
               vec($_, 2, 8) = 64  ==    4194304 00000000000000000000001000000000
               vec($_, 3, 8) = 64  == 1073741824 00000000000000000000000000000010
               vec($_, 0, 8) = 128 ==        128 00000001000000000000000000000000
               vec($_, 1, 8) = 128 ==      32768 00000000000000010000000000000000
               vec($_, 2, 8) = 128 ==    8388608 00000000000000000000000100000000
               vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001

           Behaves like wait(2) on your system: it waits for a child process
           to terminate and returns the pid of the deceased process, or "-1"
           if there are no child processes.  The status is returned in $?  and
           "${^CHILD_ERROR_NATIVE}".  Note that a return value of "-1" could
           mean that child processes are being automatically reaped, as
           described in perlipc.

           If you use wait in your handler for $SIG{CHLD} it may accidentally
           for the child created by qx() or system(). See perlipc for details.

       waitpid PID,FLAGS
           Waits for a particular child process to terminate and returns the
           pid of the deceased process, or "-1" if there is no such child
           process.  On some systems, a value of 0 indicates that there are
           processes still running.  The status is returned in $? and
           "${^CHILD_ERROR_NATIVE}".  If you say

               use POSIX ":sys_wait_h";
               do {
                   $kid = waitpid(-1, WNOHANG);
               } while $kid > 0;

           then you can do a non-blocking wait for all pending zombie
           processes.  Non-blocking wait is available on machines supporting
           either the waitpid(2) or wait4(2) syscalls.  However, waiting for a
           particular pid with FLAGS of 0 is implemented everywhere.  (Perl
           emulates the system call by remembering the status values of

               return unless defined wantarray; # don't bother doing more
               my @a = complex_calculation();
               return wantarray ? @a : "@a";

           "wantarray()"'s result is unspecified in the top level of a file,
           in a "BEGIN", "UNITCHECK", "CHECK", "INIT" or "END" block, or in a
           "DESTROY" method.

           This function should have been named wantlist() instead.

       warn LIST
           Prints the value of LIST to STDERR.  If the last element of LIST
           does not end in a newline, it appends the same file/line number
           text as "die" does.

           If the output is empty and $@ already contains a value (typically
           from a previous eval) that value is used after appending
           "\t...caught" to $@.  This is useful for staying almost, but not
           entirely similar to "die".

           If $@ is empty then the string "Warning: Something's wrong" is

           No message is printed if there is a $SIG{__WARN__} handler
           installed.  It is the handler's responsibility to deal with the
           message as it sees fit (like, for instance, converting it into a
           "die").  Most handlers must therefore arrange to actually display
           the warnings that they are not prepared to deal with, by calling
           "warn" again in the handler.  Note that this is quite safe and will
           not produce an endless loop, since "__WARN__" hooks are not called
           from inside one.

           You will find this behavior is slightly different from that of
           $SIG{__DIE__} handlers (which don't suppress the error text, but
           can instead call "die" again to change it).

           Using a "__WARN__" handler provides a powerful way to silence all
           warnings (even the so-called mandatory ones).  An example:

               # wipe out *all* compile-time warnings
               BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
               my $foo = 10;
               my $foo = 20;          # no warning about duplicate my $foo,
                                      # but hey, you asked for it!
               # no compile-time or run-time warnings before here
               $DOWARN = 1;

               # run-time warnings enabled after here
               warn "\$foo is alive and $foo!";     # does show up

           See perlvar for details on setting %SIG entries and for more
           examples.  See the Carp module for other kinds of warnings using
           its carp() and cluck() functions.
                       say "I like apples."
                   when (/oranges?/) {
                       say "I don't like oranges."
                   default {
                       say "I don't like anything"

               # require 5.14 for when as statement modifier
               use v5.14;
               foreach (@fruits) {
                   say "I like apples."        when /apples?/;
                   say "I don't like oranges." when /oranges?;
                   default { say "I don't like anything" }

               use v5.10;
               given ($fruit) {
                   when (/apples?/) {
                       say "I like apples."
                   when (/oranges?/) {
                       say "I don't like oranges."
                   default {
                       say "I don't like anything"

           See "Switch statements" in perlsyn for detailed information.

       write FILEHANDLE
       write EXPR
           Writes a formatted record (possibly multi-line) to the specified
           FILEHANDLE, using the format associated with that file.  By default
           the format for a file is the one having the same name as the
           filehandle, but the format for the current output channel (see the
           "select" function) may be set explicitly by assigning the name of
           the format to the $~ variable.

           Top of form processing is handled automatically:  if there is
           insufficient room on the current page for the formatted record, the
           page is advanced by writing a form feed, a special top-of-page
           format is used to format the new page header before the record is
           written.  By default, the top-of-page format is the name of the
           filehandle with "_TOP" appended. This would be a problem with
           autovivified filehandles, but it may be dynamically set to the
           format of your choice by assigning the name to the $^ variable
           while that filehandle is selected.  The number of lines remaining
           on the current page is in variable "$-", which can be set to 0 to
           force a new page.
           Quote-like Operators" in perlop.

perl v5.14.2                      2016-03-01                       PERLFUNC(1)
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