perlvar


DESCRIPTION
   The Syntax of Variable Names
       Variable names in Perl can have several formats. Usually, they must
       begin with a letter or underscore, in which case they can be
       arbitrarily long (up to an internal limit of 251 characters) and may
       contain letters, digits, underscores, or the special sequence "::" or
       "'". In this case, the part before the last "::" or "'" is taken to be
       a package qualifier; see perlmod.

       Perl variable names may also be a sequence of digits or a single
       punctuation or control character. These names are all reserved for
       special uses by Perl; for example, the all-digits names are used to
       hold data captured by backreferences after a regular expression match.
       Perl has a special syntax for the single-control-character names: It
       understands "^X" (caret "X") to mean the control-"X" character. For
       example, the notation $^W (dollar-sign caret "W") is the scalar
       variable whose name is the single character control-"W". This is better
       than typing a literal control-"W" into your program.

       Since Perl 5.6, Perl variable names may be alphanumeric strings that
       begin with control characters (or better yet, a caret).  These
       variables must be written in the form "${^Foo}"; the braces are not
       optional. "${^Foo}" denotes the scalar variable whose name is a
       control-"F" followed by two "o"'s. These variables are reserved for
       future special uses by Perl, except for the ones that begin with "^_"
       (control-underscore or caret-underscore). No control-character name
       that begins with "^_" will acquire a special meaning in any future
       version of Perl; such names may therefore be used safely in programs.
       $^_ itself, however, is reserved.

       Perl identifiers that begin with digits, control characters, or
       punctuation characters are exempt from the effects of the "package"
       declaration and are always forced to be in package "main"; they are
       also exempt from "strict 'vars'" errors. A few other names are also
       exempt in these ways:

               ENV      STDIN
               INC      STDOUT
               ARGV     STDERR
               ARGVOUT
               SIG

       In particular, the special "${^_XYZ}" variables are always taken to be
       in package "main", regardless of any "package" declarations presently
       in scope.

SPECIAL VARIABLES
       The following names have special meaning to Perl. Most punctuation
       names have reasonable mnemonics, or analogs in the shells.
       Nevertheless, if you wish to use long variable names, you need only
       say:

               use English;

       $^T), although $_ and @_ move up to the top of the pile.  For variables
       with the same identifier, we list it in order of scalar, array, hash,
       and bareword.

   General Variables
       $ARG
       $_      The default input and pattern-searching space. The following
               pairs are equivalent:

                       while (<>) {...}    # equivalent only in while!
                       while (defined($_ = <>)) {...}

                       /^Subject:/
                       $_ =~ /^Subject:/

                       tr/a-z/A-Z/
                       $_ =~ tr/a-z/A-Z/

                       chomp
                       chomp($_)

               Here are the places where Perl will assume $_ even if you don't
               use it:

               o  The following functions use $_ as a default argument:

                  abs, alarm, chomp, chop, chr, chroot, cos, defined, eval,
                  exp, glob, hex, int, lc, lcfirst, length, log, lstat, mkdir,
                  oct, ord, pos, print, quotemeta, readlink, readpipe, ref,
                  require, reverse (in scalar context only), rmdir, sin, split
                  (on its second argument), sqrt, stat, study, uc, ucfirst,
                  unlink, unpack.

               o  All file tests ("-f", "-d") except for "-t", which defaults
                  to STDIN.  See "-X" in perlfunc

               o  The pattern matching operations "m//", "s///" and "tr///"
                  (aka "y///") when used without an "=~" operator.

               o  The default iterator variable in a "foreach" loop if no
                  other variable is supplied.

               o  The implicit iterator variable in the "grep()" and "map()"
                  functions.

               o  The implicit variable of "given()".

               o  The default place to put an input record when a "<FH>"
                  operation's result is tested by itself as the sole criterion
                  of a "while" test. Outside a "while" test, this will not
                  happen.

               As $_ is a global variable, this may lead in some cases to
               unwanted side-effects. As of perl 5.9.1, you can now use a
               See perlsub.

       $LIST_SEPARATOR
       $"      When an array or an array slice is interpolated into a double-
               quoted string or a similar context such as "/.../", its
               elements are separated by this value. Default is a space. For
               example, this:

                       print "The array is: @array\n";

               is equivalent to this:

                       print "The array is: " . join($", @array) . "\n";

               Mnemonic: works in double-quoted context.

       $PROCESS_ID
       $PID
       $$      The process number of the Perl running this script. You should
               consider this variable read-only, although it will be altered
               across "fork()" calls.

               Note for Linux users: on Linux, the C functions "getpid()" and
               "getppid()" return different values from different threads. In
               order to be portable, this behavior is not reflected by $$,
               whose value remains consistent across threads. If you want to
               call the underlying "getpid()", you may use the CPAN module
               "Linux::Pid".

               Mnemonic: same as shells.

       $REAL_GROUP_ID
       $GID
       $(      The real gid of this process. If you are on a machine that
               supports membership in multiple groups simultaneously, gives a
               space separated list of groups you are in. The first number is
               the one returned by "getgid()", and the subsequent ones by
               "getgroups()", one of which may be the same as the first
               number.

               However, a value assigned to $( must be a single number used to
               set the real gid. So the value given by $( should not be
               assigned back to $( without being forced numeric, such as by
               adding zero. Note that this is different to the effective gid
               ($)) which does take a list.

               You can change both the real gid and the effective gid at the
               same time by using "POSIX::setgid()". Changes to $( require a
               check to $!  to detect any possible errors after an attempted
               change.

               Mnemonic: parentheses are used to group things. The real gid is
               the group you left, if you're running setgid.

               the effect of an empty list for "setgroups()", just repeat the
               new effective gid; that is, to force an effective gid of 5 and
               an effectively empty "setgroups()" list, say " $) = "5 5" ".

               You can change both the effective gid and the real gid at the
               same time by using "POSIX::setgid()" (use only a single numeric
               argument).  Changes to $) require a check to $! to detect any
               possible errors after an attempted change.

               $<, $>, $( and $) can be set only on machines that support the
               corresponding set[re][ug]id() routine. $( and $) can be swapped
               only on machines supporting "setregid()".

               Mnemonic: parentheses are used to group things. The effective
               gid is the group that's right for you, if you're running
               setgid.

       $PROGRAM_NAME
       $0      Contains the name of the program being executed.

               On some (but not all) operating systems assigning to $0
               modifies the argument area that the "ps" program sees. On some
               platforms you may have to use special "ps" options or a
               different "ps" to see the changes. Modifying the $0 is more
               useful as a way of indicating the current program state than it
               is for hiding the program you're running.

               Note that there are platform-specific limitations on the
               maximum length of $0. In the most extreme case it may be
               limited to the space occupied by the original $0.

               In some platforms there may be arbitrary amount of padding, for
               example space characters, after the modified name as shown by
               "ps".  In some platforms this padding may extend all the way to
               the original length of the argument area, no matter what you do
               (this is the case for example with Linux 2.2).

               Note for BSD users: setting $0 does not completely remove
               "perl" from the ps(1) output. For example, setting $0 to
               "foobar" may result in "perl: foobar (perl)" (whether both the
               "perl: " prefix and the " (perl)" suffix are shown depends on
               your exact BSD variant and version). This is an operating
               system feature, Perl cannot help it.

               In multithreaded scripts Perl coordinates the threads so that
               any thread may modify its copy of the $0 and the change becomes
               visible to ps(1) (assuming the operating system plays along).
               Note that the view of $0 the other threads have will not change
               since they have their own copies of it.

               If the program has been given to perl via the switches "-e" or
               "-E", $0 will contain the string "-e".

               On Linux as of perl 5.14 the legacy process name will be set
               If you refer to a hash element as

                       $foo{$a,$b,$c}

               it really means

                       $foo{join($;, $a, $b, $c)}

               But don't put

                       @foo{$a,$b,$c}  # a slice--note the @

               which means

                       ($foo{$a},$foo{$b},$foo{$c})

               Default is "\034", the same as SUBSEP in awk. If your keys
               contain binary data there might not be any safe value for $;.

               Consider using "real" multidimensional arrays as described in
               perllol.

               Mnemonic: comma (the syntactic subscript separator) is a semi-
               semicolon.

       $REAL_USER_ID
       $UID
       $<      The real uid of this process. You can change both the real uid
               and the effective uid at the same time by using
               "POSIX::setuid()". Since changes to $< require a system call,
               check $! after a change attempt to detect any possible errors.

               Mnemonic: it's the uid you came from, if you're running setuid.

       $EFFECTIVE_USER_ID
       $EUID
       $>      The effective uid of this process. For example:

                       $< = $>;            # set real to effective uid
                       ($<,$>) = ($>,$<);  # swap real and effective uids

               You can change both the effective uid and the real uid at the
               same time by using "POSIX::setuid()". Changes to $> require a
               check to $! to detect any possible errors after an attempted
               change.

               $< and $> can be swapped only on machines supporting
               "setreuid()".

               Mnemonic: it's the uid you went to, if you're running setuid.

       $a
       $b      Special package variables when using "sort()", see "sort" in
               perlfunc.  Because of this specialness $a and $b don't need to
               This variable was added in Perl 5.6.

       $DEBUGGING
       $^D     The current value of the debugging flags. May be read or set.
               Like its command-line equivalent, you can use numeric or
               symbolic values, eg "$^D = 10" or "$^D = "st"".

               Mnemonic: value of -D switch.

       ${^ENCODING}
               The object reference to the "Encode" object that is used to
               convert the source code to Unicode. Thanks to this variable
               your Perl script does not have to be written in UTF-8. Default
               is undef. The direct manipulation of this variable is highly
               discouraged.

               This variable was added in Perl 5.8.2.

       %ENV    The hash %ENV contains your current environment. Setting a
               value in "ENV" changes the environment for any child processes
               you subsequently "fork()" off.

       $SYSTEM_FD_MAX
       $^F     The maximum system file descriptor, ordinarily 2. System file
               descriptors are passed to "exec()"ed processes, while higher
               file descriptors are not. Also, during an "open()", system file
               descriptors are preserved even if the "open()" fails (ordinary
               file descriptors are closed before the "open()" is attempted).
               The close-on-exec status of a file descriptor will be decided
               according to the value of $^F when the corresponding file,
               pipe, or socket was opened, not the time of the "exec()".

       @F      The array @F contains the fields of each line read in when
               autosplit mode is turned on. See perlrun for the -a switch.
               This array is package-specific, and must be declared or given a
               full package name if not in package main when running under
               "strict 'vars'".

       ${^GLOBAL_PHASE}
               The current phase of the perl interpreter.

               Possible values are:

               CONSTRUCT
                       The "PerlInterpreter*" is being constructed via
                       "perl_construct". This value is mostly there for
                       completeness and for use via the underlying C variable
                       "PL_phase". It's not really possible for Perl code to
                       be executed unless construction of the interpreter is
                       finished.

               START   This is the global compile-time. That includes,
                       basically, every "BEGIN" block executed directly or
                       indirectly from during the compile-time of the top-

               INIT    Similar to "CHECK", but for "INIT"-blocks, not "CHECK"
                       blocks.

               RUN     The main run-time, i.e. the execution of
                       "PL_main_root".

               END     Execution of any "END" blocks.

               DESTRUCT
                       Global destruction.

               Also note that there's no value for UNITCHECK-blocks. That's
               because those are run for each compilation unit individually,
               and therefore is not a global interpreter phase.

               Not every program has to go through each of the possible
               phases, but transition from one phase to another can only
               happen in the order described in the above list.

               An example of all of the phases Perl code can see:

                   BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }

                   INIT  { print "init-time: ${^GLOBAL_PHASE}\n" }

                   CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }

                   {
                       package Print::Phase;

                       sub new {
                           my ($class, $time) = @_;
                           return bless \$time, $class;
                       }

                       sub DESTROY {
                           my $self = shift;
                           print "$$self: ${^GLOBAL_PHASE}\n";
                       }
                   }

                   print "run-time: ${^GLOBAL_PHASE}\n";

                   my $runtime = Print::Phase->new(
                       "lexical variables are garbage collected before END"
                   );

                   END   { print "end-time: ${^GLOBAL_PHASE}\n" }

                   our $destruct = Print::Phase->new(
                       "package variables are garbage collected after END"
                   );

               This will print out
               availability, behavior, and contents are subject to change
               without notice.

               This variable contains compile-time hints for the Perl
               interpreter. At the end of compilation of a BLOCK the value of
               this variable is restored to the value when the interpreter
               started to compile the BLOCK.

               When perl begins to parse any block construct that provides a
               lexical scope (e.g., eval body, required file, subroutine body,
               loop body, or conditional block), the existing value of $^H is
               saved, but its value is left unchanged.  When the compilation
               of the block is completed, it regains the saved value.  Between
               the points where its value is saved and restored, code that
               executes within BEGIN blocks is free to change the value of
               $^H.

               This behavior provides the semantic of lexical scoping, and is
               used in, for instance, the "use strict" pragma.

               The contents should be an integer; different bits of it are
               used for different pragmatic flags. Here's an example:

                       sub add_100 { $^H |= 0x100 }

                       sub foo {
                               BEGIN { add_100() }
                               bar->baz($boon);
                       }

               Consider what happens during execution of the BEGIN block. At
               this point the BEGIN block has already been compiled, but the
               body of "foo()" is still being compiled. The new value of $^H
               will therefore be visible only while the body of "foo()" is
               being compiled.

               Substitution of "BEGIN { add_100() }" block with:

                       BEGIN { require strict; strict->import('vars') }

               demonstrates how "use strict 'vars'" is implemented. Here's a
               conditional version of the same lexical pragma:

                       BEGIN { require strict; strict->import('vars') if $condition }

               This variable was added in Perl 5.003.

       %^H     The "%^H" hash provides the same scoping semantic as $^H. This
               makes it useful for implementation of lexically scoped pragmas.
               See perlpragma.

               This variable was added in Perl 5.6.

       @INC    The array @INC contains the list of places that the "do EXPR",
               You can also insert hooks into the file inclusion system by
               putting Perl code directly into @INC. Those hooks may be
               subroutine references, array references or blessed objects. See
               "require" in perlfunc for details.

       %INC    The hash %INC contains entries for each filename included via
               the "do", "require", or "use" operators. The key is the
               filename you specified (with module names converted to
               pathnames), and the value is the location of the file found.
               The "require" operator uses this hash to determine whether a
               particular file has already been included.

               If the file was loaded via a hook (e.g. a subroutine reference,
               see "require" in perlfunc for a description of these hooks),
               this hook is by default inserted into %INC in place of a
               filename. Note, however, that the hook may have set the %INC
               entry by itself to provide some more specific info.

       $INPLACE_EDIT
       $^I     The current value of the inplace-edit extension. Use "undef" to
               disable inplace editing.

               Mnemonic: value of -i switch.

       $^M     By default, running out of memory is an untrappable, fatal
               error.  However, if suitably built, Perl can use the contents
               of $^M as an emergency memory pool after "die()"ing. Suppose
               that your Perl were compiled with "-DPERL_EMERGENCY_SBRK" and
               used Perl's malloc.  Then

                       $^M = 'a' x (1 << 16);

               would allocate a 64K buffer for use in an emergency. See the
               INSTALL file in the Perl distribution for information on how to
               add custom C compilation flags when compiling perl. To
               discourage casual use of this advanced feature, there is no
               English long name for this variable.

               This variable was added in Perl 5.004.

       $OSNAME
       $^O     The name of the operating system under which this copy of Perl
               was built, as determined during the configuration process. For
               examples see "PLATFORMS" in perlport.

               The value is identical to $Config{'osname'}. See also Config
               and the -V command-line switch documented in perlrun.

               In Windows platforms, $^O is not very helpful: since it is
               always "MSWin32", it doesn't tell the difference between
               95/98/ME/NT/2000/XP/CE/.NET. Use "Win32::GetOSName()" or
               Win32::GetOSVersion() (see Win32 and perlport) to distinguish
               between the variants.


               0x01  Debug subroutine enter/exit.

               0x02  Line-by-line debugging. Causes "DB::DB()" subroutine to
                     be called for each statement executed. Also causes saving
                     source code lines (like 0x400).

               0x04  Switch off optimizations.

               0x08  Preserve more data for future interactive inspections.

               0x10  Keep info about source lines on which a subroutine is
                     defined.

               0x20  Start with single-step on.

               0x40  Use subroutine address instead of name when reporting.

               0x80  Report "goto &subroutine" as well.

               0x100 Provide informative "file" names for evals based on the
                     place they were compiled.

               0x200 Provide informative names to anonymous subroutines based
                     on the place they were compiled.

               0x400 Save source code lines into "@{"_<$filename"}".

               Some bits may be relevant at compile-time only, some at run-
               time only. This is a new mechanism and the details may change.
               See also perldebguts.

       %SIG    The hash %SIG contains signal handlers for signals. For
               example:

                       sub handler {   # 1st argument is signal name
                               my($sig) = @_;
                               print "Caught a SIG$sig--shutting down\n";
                               close(LOG);
                               exit(0);
                               }

                       $SIG{'INT'}  = \&handler;
                       $SIG{'QUIT'} = \&handler;
                       ...
                       $SIG{'INT'}  = 'DEFAULT';   # restore default action
                       $SIG{'QUIT'} = 'IGNORE';    # ignore SIGQUIT

               Using a value of 'IGNORE' usually has the effect of ignoring
               the signal, except for the "CHLD" signal. See perlipc for more
               about this special case.

               Here are some other examples:

               The default delivery policy of signals changed in Perl 5.8.0
               from immediate (also known as "unsafe") to deferred, also known
               as "safe signals". See perlipc for more information.

               Certain internal hooks can be also set using the %SIG hash. The
               routine indicated by $SIG{__WARN__} is called when a warning
               message is about to be printed. The warning message is passed
               as the first argument. The presence of a "__WARN__" hook causes
               the ordinary printing of warnings to "STDERR" to be suppressed.
               You can use this to save warnings in a variable, or turn
               warnings into fatal errors, like this:

                       local $SIG{__WARN__} = sub { die $_[0] };
                       eval $proggie;

               As the 'IGNORE' hook is not supported by "__WARN__", you can
               disable warnings using the empty subroutine:

                       local $SIG{__WARN__} = sub {};

               The routine indicated by $SIG{__DIE__} is called when a fatal
               exception is about to be thrown. The error message is passed as
               the first argument. When a "__DIE__" hook routine returns, the
               exception processing continues as it would have in the absence
               of the hook, unless the hook routine itself exits via a "goto",
               a loop exit, or a "die()". The "__DIE__" handler is explicitly
               disabled during the call, so that you can die from a "__DIE__"
               handler. Similarly for "__WARN__".

               Due to an implementation glitch, the $SIG{__DIE__} hook is
               called even inside an "eval()". Do not use this to rewrite a
               pending exception in $@, or as a bizarre substitute for
               overriding "CORE::GLOBAL::die()". This strange action at a
               distance may be fixed in a future release so that $SIG{__DIE__}
               is only called if your program is about to exit, as was the
               original intent. Any other use is deprecated.

               "__DIE__"/"__WARN__" handlers are very special in one respect:
               they may be called to report (probable) errors found by the
               parser. In such a case the parser may be in inconsistent state,
               so any attempt to evaluate Perl code from such a handler will
               probably result in a segfault. This means that warnings or
               errors that result from parsing Perl should be used with
               extreme caution, like this:

                       require Carp if defined $^S;
                       Carp::confess("Something wrong") if defined &Carp::confess;
                       die "Something wrong, but could not load Carp to give backtrace...
                          To see backtrace try starting Perl with -MCarp switch";

               Here the first line will load "Carp" unless it is the parser
               who called the handler. The second line will print backtrace
               and die if "Carp" was available. The third line will be
               executed only if "Carp" was not available.
               the epoch (beginning of 1970). The values returned by the -M,
               -A, and -C filetests are based on this value.

       ${^TAINT}
               Reflects if taint mode is on or off. 1 for on (the program was
               run with -T), 0 for off, -1 when only taint warnings are
               enabled (i.e. with -t or -TU).

               This variable is read-only.

               This variable was added in Perl 5.8.

       ${^UNICODE}
               Reflects certain Unicode settings of Perl. See perlrun
               documentation for the "-C" switch for more information about
               the possible values.

               This variable is set during Perl startup and is thereafter
               read-only.

               This variable was added in Perl 5.8.2.

       ${^UTF8CACHE}
               This variable controls the state of the internal UTF-8 offset
               caching code.  1 for on (the default), 0 for off, -1 to debug
               the caching code by checking all its results against linear
               scans, and panicking on any discrepancy.

               This variable was added in Perl 5.8.9.

       ${^UTF8LOCALE}
               This variable indicates whether a UTF-8 locale was detected by
               perl at startup. This information is used by perl when it's in
               adjust-utf8ness-to-locale mode (as when run with the "-CL"
               command-line switch); see perlrun for more info on this.

               This variable was added in Perl 5.8.8.

       $PERL_VERSION
       $^V     The revision, version, and subversion of the Perl interpreter,
               represented as a "version" object.

               This variable first appeared in perl 5.6.0; earlier versions of
               perl will see an undefined value. Before perl 5.10.0 $^V was
               represented as a v-string.

               $^V can be used to determine whether the Perl interpreter
               executing a script is in the right range of versions. For
               example:

                       warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1

               To convert $^V into its string representation use "sprintf()"'s
               "%vd" conversion:

       ${^WIN32_SLOPPY_STAT}
               If this variable is set to a true value, then "stat()" on
               Windows will not try to open the file. This means that the link
               count cannot be determined and file attributes may be out of
               date if additional hardlinks to the file exist. On the other
               hand, not opening the file is considerably faster, especially
               for files on network drives.

               This variable could be set in the sitecustomize.pl file to
               configure the local Perl installation to use "sloppy" "stat()"
               by default. See the documentation for -f in perlrun for more
               information about site customization.

               This variable was added in Perl 5.10.

       $EXECUTABLE_NAME
       $^X     The name used to execute the current copy of Perl, from C's
               "argv[0]" or (where supported) /proc/self/exe.

               Depending on the host operating system, the value of $^X may be
               a relative or absolute pathname of the perl program file, or
               may be the string used to invoke perl but not the pathname of
               the perl program file. Also, most operating systems permit
               invoking programs that are not in the PATH environment
               variable, so there is no guarantee that the value of $^X is in
               PATH. For VMS, the value may or may not include a version
               number.

               You usually can use the value of $^X to re-invoke an
               independent copy of the same perl that is currently running,
               e.g.,

                       @first_run = `$^X -le "print int rand 100 for 1..100"`;

               But recall that not all operating systems support forking or
               capturing of the output of commands, so this complex statement
               may not be portable.

               It is not safe to use the value of $^X as a path name of a
               file, as some operating systems that have a mandatory suffix on
               executable files do not require use of the suffix when invoking
               a command. To convert the value of $^X to a path name, use the
               following statements:

                       # Build up a set of file names (not command names).
                       use Config;
                       my $this_perl = $^X;
                       if ($^O ne 'VMS') {
                               $this_perl .= $Config{_exe}
                                 unless $this_perl =~ m/$Config{_exe}$/i;
                               }

               Because many operating systems permit anyone with read access
                               }

   Variables related to regular expressions
       Most of the special variables related to regular expressions are side
       effects. Perl sets these variables when it has a successful match, so
       you should check the match result before using them. For instance:

               if( /P(A)TT(ER)N/ ) {
                       print "I found $1 and $2\n";
                       }

       These variables are read-only and dynamically-scoped, unless we note
       otherwise.

       The dynamic nature of the regular expression variables means that their
       value is limited to the block that they are in, as demonstrated by this
       bit of code:

               my $outer = 'Wallace and Grommit';
               my $inner = 'Mutt and Jeff';

               my $pattern = qr/(\S+) and (\S+)/;

               sub show_n { print "\$1 is $1; \$2 is $2\n" }

               {
               OUTER:
                       show_n() if $outer =~ m/$pattern/;

                       INNER: {
                               show_n() if $inner =~ m/$pattern/;
                               }

                       show_n();
               }

       The output shows that while in the "OUTER" block, the values of $1 and
       $2 are from the match against $outer. Inside the "INNER" block, the
       values of $1 and $2 are from the match against $inner, but only until
       the end of the block (i.e. the dynamic scope). After the "INNER" block
       completes, the values of $1 and $2 return to the values for the match
       against $outer even though we have not made another match:

               $1 is Wallace; $2 is Grommit
               $1 is Mutt; $2 is Jeff
               $1 is Wallace; $2 is Grommit

       Due to an unfortunate accident of Perl's implementation, "use English"
       imposes a considerable performance penalty on all regular expression
       matches in a program because it uses the "$`", $&, and "$'", regardless
       of whether they occur in the scope of "use English". For that reason,
       saying "use English" in libraries is strongly discouraged unless you
       import it without the match variables:

               counting patterns matched in nested blocks that have been
               exited already.

               These variables are read-only and dynamically-scoped.

               Mnemonic: like \digits.

       $MATCH
       $&      The string matched by the last successful pattern match (not
               counting any matches hidden within a BLOCK or "eval()" enclosed
               by the current BLOCK).

               The use of this variable anywhere in a program imposes a
               considerable performance penalty on all regular expression
               matches. To avoid this penalty, you can extract the same
               substring by using "@-". Starting with Perl 5.10, you can use
               the </p> match flag and the "${^MATCH}" variable to do the same
               thing for particular match operations.

               This variable is read-only and dynamically-scoped.

               Mnemonic: like "&" in some editors.

       ${^MATCH}
               This is similar to $& ($MATCH) except that it does not incur
               the performance penalty associated with that variable, and is
               only guaranteed to return a defined value when the pattern was
               compiled or executed with the "/p" modifier.

               This variable was added in Perl 5.10.

               This variable is read-only and dynamically-scoped.

       $PREMATCH
       $`      The string preceding whatever was matched by the last
               successful pattern match, not counting any matches hidden
               within a BLOCK or "eval" enclosed by the current BLOCK.

               The use of this variable anywhere in a program imposes a
               considerable performance penalty on all regular expression
               matches. To avoid this penalty, you can extract the same
               substring by using "@-". Starting with Perl 5.10, you can use
               the </p> match flag and the "${^PREMATCH}" variable to do the
               same thing for particular match operations.

               This variable is read-only and dynamically-scoped.

               Mnemonic: "`" often precedes a quoted string.

       ${^PREMATCH}
               This is similar to "$`" ($PREMATCH) except that it does not
               incur the performance penalty associated with that variable,
               and is only guaranteed to return a defined value when the
               pattern was compiled or executed with the "/p" modifier.
                       /def/;
                       print "$`:$&:$'\n";     # prints abc:def:ghi

               The use of this variable anywhere in a program imposes a
               considerable performance penalty on all regular expression
               matches.  To avoid this penalty, you can extract the same
               substring by using "@-". Starting with Perl 5.10, you can use
               the </p> match flag and the "${^POSTMATCH}" variable to do the
               same thing for particular match operations.

               This variable is read-only and dynamically-scoped.

               Mnemonic: "'" often follows a quoted string.

       ${^POSTMATCH}
               This is similar to "$'" ($POSTMATCH) except that it does not
               incur the performance penalty associated with that variable,
               and is only guaranteed to return a defined value when the
               pattern was compiled or executed with the "/p" modifier.

               This variable was added in Perl 5.10.

               This variable is read-only and dynamically-scoped.

       $LAST_PAREN_MATCH
       $+      The text matched by the last bracket of the last successful
               search pattern.  This is useful if you don't know which one of
               a set of alternative patterns matched. For example:

                       /Version: (.*)|Revision: (.*)/ && ($rev = $+);

               This variable is read-only and dynamically-scoped.

               Mnemonic: be positive and forward looking.

       $LAST_SUBMATCH_RESULT
       $^N     The text matched by the used group most-recently closed (i.e.
               the group with the rightmost closing parenthesis) of the last
               successful search pattern.

               This is primarily used inside "(?{...})" blocks for examining
               text recently matched. For example, to effectively capture text
               to a variable (in addition to $1, $2, etc.), replace "(...)"
               with

                       (?:(...)(?{ $var = $^N }))

               By setting and then using $var in this way relieves you from
               having to worry about exactly which numbered set of parentheses
               they are.

               This variable was added in Perl 5.8.

               Mnemonic: the (possibly) Nested parenthesis that most recently
               given for the "@-" variable.

               This variable was added in Perl 5.6.

       %LAST_PAREN_MATCH
       %+      Similar to "@+", the "%+" hash allows access to the named
               capture buffers, should they exist, in the last successful
               match in the currently active dynamic scope.

               For example, $+{foo} is equivalent to $1 after the following
               match:

                       'foo' =~ /(?<foo>foo)/;

               The keys of the "%+" hash list only the names of buffers that
               have captured (and that are thus associated to defined values).

               The underlying behaviour of "%+" is provided by the
               Tie::Hash::NamedCapture module.

               Note: "%-" and "%+" are tied views into a common internal hash
               associated with the last successful regular expression.
               Therefore mixing iterative access to them via "each" may have
               unpredictable results.  Likewise, if the last successful match
               changes, then the results may be surprising.

               This variable was added in Perl 5.10.

               This variable is read-only and dynamically-scoped.

       @LAST_MATCH_START
       @-      "$-[0]" is the offset of the start of the last successful
               match.  "$-["n"]" is the offset of the start of the substring
               matched by n-th subpattern, or undef if the subpattern did not
               match.

               Thus, after a match against $_, $& coincides with "substr $_,
               $-[0], $+[0] - $-[0]". Similarly, $n coincides with "substr $_,
               $-[n], $+[n] - $-[n]" if "$-[n]" is defined, and $+ coincides
               with "substr $_, $-[$#-], $+[$#-] - $-[$#-]". One can use "$#-"
               to find the last matched subgroup in the last successful match.
               Contrast with $#+, the number of subgroups in the regular
               expression. Compare with "@+".

               This array holds the offsets of the beginnings of the last
               successful submatches in the currently active dynamic scope.
               "$-[0]" is the offset into the string of the beginning of the
               entire match. The nth element of this array holds the offset of
               the nth submatch, so "$-[1]" is the offset where $1 begins,
               "$-[2]" the offset where $2 begins, and so on.

               After a match against some variable $var:

               "$`" is the same as "substr($var, 0, $-[0])"
               regular expression, it associates a reference to an array
               containing the list of values captured by all buffers with that
               name (should there be several of them), in the order where they
               appear.

               Here's an example:

                   if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
                       foreach my $bufname (sort keys %-) {
                           my $ary = $-{$bufname};
                           foreach my $idx (0..$#$ary) {
                               print "\$-{$bufname}[$idx] : ",
                                     (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
                                     "\n";
                           }
                       }
                   }

               would print out:

                       $-{A}[0] : '1'
                       $-{A}[1] : '3'
                       $-{B}[0] : '2'
                       $-{B}[1] : '4'

               The keys of the "%-" hash correspond to all buffer names found
               in the regular expression.

               The behaviour of "%-" is implemented via the
               Tie::Hash::NamedCapture module.

               Note: "%-" and "%+" are tied views into a common internal hash
               associated with the last successful regular expression.
               Therefore mixing iterative access to them via "each" may have
               unpredictable results.  Likewise, if the last successful match
               changes, then the results may be surprising.

               This variable was added in Perl 5.10

               This variable is read-only and dynamically-scoped.

       $LAST_REGEXP_CODE_RESULT
       $^R     The result of evaluation of the last successful "(?{ code })"
               regular expression assertion (see perlre). May be written to.

               This variable was added in Perl 5.005.

       ${^RE_DEBUG_FLAGS}
               The current value of the regex debugging flags. Set to 0 for no
               debug output even when the "re 'debug'" module is loaded. See
               re for details.

               This variable was added in Perl 5.10.


   Variables related to filehandles
       Variables that depend on the currently selected filehandle may be set
       by calling an appropriate object method on the "IO::Handle" object,
       although this is less efficient than using the regular built-in
       variables. (Summary lines below for this contain the word HANDLE.)
       First you must say

               use IO::Handle;

       after which you may use either

               method HANDLE EXPR

       or more safely,

               HANDLE->method(EXPR)

       Each method returns the old value of the "IO::Handle" attribute. The
       methods each take an optional EXPR, which, if supplied, specifies the
       new value for the "IO::Handle" attribute in question. If not supplied,
       most methods do nothing to the current value--except for "autoflush()",
       which will assume a 1 for you, just to be different.

       Because loading in the "IO::Handle" class is an expensive operation,
       you should learn how to use the regular built-in variables.

       A few of these variables are considered "read-only". This means that if
       you try to assign to this variable, either directly or indirectly
       through a reference, you'll raise a run-time exception.

       You should be very careful when modifying the default values of most
       special variables described in this document. In most cases you want to
       localize these variables before changing them, since if you don't, the
       change may affect other modules which rely on the default values of the
       special variables that you have changed. This is one of the correct
       ways to read the whole file at once:

               open my $fh, "<", "foo" or die $!;
               local $/; # enable localized slurp mode
               my $content = <$fh>;
               close $fh;

       But the following code is quite bad:

               open my $fh, "<", "foo" or die $!;
               undef $/; # enable slurp mode
               my $content = <$fh>;
               close $fh;

       since some other module, may want to read data from some file in the
       default "line mode", so if the code we have just presented has been
       executed, the global value of $/ is now changed for any other code
       running inside the same Perl interpreter.
               close $fh;

       Here is an example of how your own code can go broken:

               for ( 1..3 ){
                       $\ = "\r\n";
                       nasty_break();
                       print "$_";
               }

               sub nasty_break {
               $\ = "\f";
               # do something with $_
               }

       You probably expect this code to print the equivalent of

           "1\r\n2\r\n3\r\n"

       but instead you get:

           "1\f2\f3\f"

       Why? Because "nasty_break()" modifies "$\" without localizing it first.
       The value you set in  "nasty_break()" is still there when you return.
       The fix is to add "local()" so the value doesn't leak out of
       "nasty_break()":

               local $\ = "\f";

       It's easy to notice the problem in such a short example, but in more
       complicated code you are looking for trouble if you don't localize
       changes to the special variables.

       $ARGV   Contains the name of the current file when reading from "<>".

       @ARGV   The array @ARGV contains the command-line arguments intended
               for the script. $#ARGV is generally the number of arguments
               minus one, because $ARGV[0] is the first argument, not the
               program's command name itself. See $0 for the command name.

       ARGV    The special filehandle that iterates over command-line
               filenames in @ARGV. Usually written as the null filehandle in
               the angle operator "<>". Note that currently "ARGV" only has
               its magical effect within the "<>" operator; elsewhere it is
               just a plain filehandle corresponding to the last file opened
               by "<>". In particular, passing "\*ARGV" as a parameter to a
               function that expects a filehandle may not cause your function
               to automatically read the contents of all the files in @ARGV.

       ARGVOUT The special filehandle that points to the currently open output
               file when doing edit-in-place processing with -i. Useful when
               you have to do a lot of inserting and don't want to keep
               modifying $_. See perlrun for the -i switch.

       $INPUT_LINE_NUMBER
       $NR
       $.      Current line number for the last filehandle accessed.

               Each filehandle in Perl counts the number of lines that have
               been read from it. (Depending on the value of $/, Perl's idea
               of what constitutes a line may not match yours.)  When a line
               is read from a filehandle (via "readline()" or "<>"), or when
               "tell()" or "seek()" is called on it, $. becomes an alias to
               the line counter for that filehandle.

               You can adjust the counter by assigning to $., but this will
               not actually move the seek pointer. Localizing $. will not
               localize the filehandle's line count. Instead, it will localize
               perl's notion of which filehandle $. is currently aliased to.

               $. is reset when the filehandle is closed, but not when an open
               filehandle is reopened without an intervening "close()". For
               more details, see "I/O Operators" in perlop. Because "<>" never
               does an explicit close, line numbers increase across "ARGV"
               files (but see examples in "eof" in perlfunc).

               You can also use "HANDLE->input_line_number(EXPR)" to access
               the line counter for a given filehandle without having to worry
               about which handle you last accessed.

               Mnemonic: many programs use "." to mean the current line
               number.

       HANDLE->input_record_separator( EXPR )
       $INPUT_RECORD_SEPARATOR
       $RS
       $/      The input record separator, newline by default. This influences
               Perl's idea of what a "line" is. Works like awk's RS variable,
               including treating empty lines as a terminator if set to the
               null string (an empty line cannot contain any spaces or tabs).
               You may set it to a multi-character string to match a multi-
               character terminator, or to "undef" to read through the end of
               file. Setting it to "\n\n" means something slightly different
               than setting to "", if the file contains consecutive empty
               lines. Setting to "" will treat two or more consecutive empty
               lines as a single empty line. Setting to "\n\n" will blindly
               assume that the next input character belongs to the next
               paragraph, even if it's a newline.

                   local $/;           # enable "slurp" mode
                   local $_ = <FH>;    # whole file now here
                   s/\n[ \t]+/ /g;

               Remember: the value of $/ is a string, not a regex. awk has to
               be better for something. :-)

               Setting $/ to a reference to an integer, scalar containing an
               integer, or scalar that's convertible to an integer will
               pieces. Trying to set the record size to zero or less will
               cause reading in the (rest of the) whole file.

               On VMS, record reads are done with the equivalent of "sysread",
               so it's best not to mix record and non-record reads on the same
               file. (This is unlikely to be a problem, because any file you'd
               want to read in record mode is probably unusable in line mode.)
               Non-VMS systems do normal I/O, so it's safe to mix record and
               non-record reads of a file.

               See also "Newlines" in perlport. Also see $..

               Mnemonic: / delimits line boundaries when quoting poetry.

       Handle->output_record_separator( EXPR )
       $OUTPUT_RECORD_SEPARATOR
       $ORS
       $\      The output record separator for the print operator. If defined,
               this value is printed after the last of print's arguments.
               Default is "undef".

               Mnemonic: you set "$\" instead of adding "\n" at the end of the
               print.  Also, it's just like $/, but it's what you get "back"
               from Perl.

       HANDLE->autoflush( EXPR )
       $OUTPUT_AUTOFLUSH
       $|      If set to nonzero, forces a flush right away and after every
               write or print on the currently selected output channel.
               Default is 0 (regardless of whether the channel is really
               buffered by the system or not; $| tells you only whether you've
               asked Perl explicitly to flush after each write). STDOUT will
               typically be line buffered if output is to the terminal and
               block buffered otherwise. Setting this variable is useful
               primarily when you are outputting to a pipe or socket, such as
               when you are running a Perl program under rsh and want to see
               the output as it's happening. This has no effect on input
               buffering. See "getc" in perlfunc for that. See "select" in
               perlfunc on how to select the output channel. See also
               IO::Handle.

               Mnemonic: when you want your pipes to be piping hot.

       Variables related to formats

       The special variables for formats are a subset of those for
       filehandles. See perlform for more information about Perl's formats.

       $ACCUMULATOR
       $^A     The current value of the "write()" accumulator for "format()"
               lines.  A format contains "formline()" calls that put their
               result into $^A. After calling its format, "write()" prints out
               the contents of $^A and empties. So you never really see the
               contents of $^A unless you call "formline()" yourself and then
               Mnemonic: "%" is page number in nroff.

       HANDLE->format_lines_left(EXPR)
       $FORMAT_LINES_LEFT
       $-      The number of lines left on the page of the currently selected
               output channel.

               Mnemonic: lines_on_page - lines_printed.

       Handle->format_line_break_characters EXPR
       $FORMAT_LINE_BREAK_CHARACTERS
       $:      The current set of characters after which a string may be
               broken to fill continuation fields (starting with "^") in a
               format. The default is " \n-", to break on a space, newline, or
               a hyphen.

               Mnemonic: a "colon" in poetry is a part of a line.

       HANDLE->format_lines_per_page(EXPR)
       $FORMAT_LINES_PER_PAGE
       $=      The current page length (printable lines) of the currently
               selected output channel. The default is 60.

               Mnemonic: = has horizontal lines.

       HANDLE->format_top_name(EXPR)
       $FORMAT_TOP_NAME
       $^      The name of the current top-of-page format for the currently
               selected output channel. The default is the name of the
               filehandle with "_TOP" appended. For example, the default
               format top name for the "STDOUT" filehanlde is "STDOUT_TOP".

               Mnemonic: points to top of page.

       HANDLE->format_name(EXPR)
       $FORMAT_NAME
       $~      The name of the current report format for the currently
               selected output channel. The default format name is the same as
               the filehandle name. For example, the default format name for
               the "STDOUT" filehandle is just "STDOUT".

               Mnemonic: brother to $^.

   Error Variables
       The variables $@, $!, $^E, and $? contain information about different
       types of error conditions that may appear during execution of a Perl
       program. The variables are shown ordered by the "distance" between the
       subsystem which reported the error and the Perl process. They
       correspond to errors detected by the Perl interpreter, C library,
       operating system, or an external program, respectively.

       To illustrate the differences between these variables, consider the
       following Perl expression, which uses a single-quoted string. After
       execution of this statement, perl may have set all four special error

       $@ is set if the string to be "eval"-ed did not compile (this may
       happen if "open" or "close" were imported with bad prototypes), or if
       Perl code executed during evaluation "die()"d. In these cases the value
       of $@ is the compile error, or the argument to "die" (which will
       interpolate $! and $?). (See also Fatal, though.)

       Under a few operating systems, $^E may contain a more verbose error
       indicator, such as in this case, "CDROM tray not closed." Systems that
       do not support extended error messages leave $^E the same as $!.

       Finally, $? may be set to non-0 value if the external program
       /cdrom/install fails. The upper eight bits reflect specific error
       conditions encountered by the program (the program's "exit()" value).
       The lower eight bits reflect mode of failure, like signal death and
       core dump information. See wait(2) for details. In contrast to $! and
       $^E, which are set only if error condition is detected, the variable $?
       is set on each "wait" or pipe "close", overwriting the old value. This
       is more like $@, which on every "eval()" is always set on failure and
       cleared on success.

       For more details, see the individual descriptions at $@, $!, $^E, and
       $?.

       ${^CHILD_ERROR_NATIVE}
               The native status returned by the last pipe close, backtick
               ("``") command, successful call to "wait()" or "waitpid()", or
               from the "system()" operator. On POSIX-like systems this value
               can be decoded with the WIFEXITED, WEXITSTATUS, WIFSIGNALED,
               WTERMSIG, WIFSTOPPED, WSTOPSIG and WIFCONTINUED functions
               provided by the POSIX module.

               Under VMS this reflects the actual VMS exit status; i.e. it is
               the same as $? when the pragma "use vmsish 'status'" is in
               effect.

               This variable was added in Perl 5.8.9.

       $EXTENDED_OS_ERROR
       $^E     Error information specific to the current operating system. At
               the moment, this differs from $! under only VMS, OS/2, and
               Win32 (and for MacPerl). On all other platforms, $^E is always
               just the same as $!.

               Under VMS, $^E provides the VMS status value from the last
               system error. This is more specific information about the last
               system error than that provided by $!. This is particularly
               important when $!  is set to EVMSERR.

               Under OS/2, $^E is set to the error code of the last call to
               OS/2 API either via CRT, or directly from perl.

               Under Win32, $^E always returns the last error information
               reported by the Win32 call "GetLastError()" which describes the

       $EXCEPTIONS_BEING_CAUGHT
       $^S     Current state of the interpreter.

                       $^S         State
                       ---------   -------------------
                       undef       Parsing module/eval
                       true (1)    Executing an eval
                       false (0)   Otherwise

               The first state may happen in $SIG{__DIE__} and $SIG{__WARN__}
               handlers.

               This variable was added in Perl 5.004.

       $WARNING
       $^W     The current value of the warning switch, initially true if -w
               was used, false otherwise, but directly modifiable.

               See also warnings.

               Mnemonic: related to the -w switch.

       ${^WARNING_BITS}
               The current set of warning checks enabled by the "use warnings"
               pragma.  See the documentation of "warnings" for more details.

               This variable was added in Perl 5.10.

       $OS_ERROR
       $ERRNO
       $!      If used numerically, yields the current value of the C "errno"
               variable, or in other words, if a system or library call fails,
               it sets this variable. This means that the value of $! is
               meaningful only immediately after a failure:

                       if (open my $fh, "<", $filename) {
                               # Here $! is meaningless.
                               ...
                   }
                   else {
                               # ONLY here is $! meaningful.
                               ...
                               # Already here $! might be meaningless.
                   }
                   # Since here we might have either success or failure,
                   # here $! is meaningless.

               The meaningless stands for anything: zero, non-zero, "undef". A
               successful system or library call does not set the variable to
               zero.

               If used as a string, yields the corresponding system error
               string. You can assign a number to $! to set errno if, for
               instance, you want "$!" to return the string for error n, or
               on your system, use "exists $!{the_key}"; for a list of legal
               keys, use "keys %!". See Errno for more information, and also
               see "$!".

               This variable was added in Perl 5.005.

       $CHILD_ERROR
       $?      The status returned by the last pipe close, backtick ("``")
               command, successful call to "wait()" or "waitpid()", or from
               the "system()" operator. This is just the 16-bit status word
               returned by the traditional Unix "wait()" system call (or else
               is made up to look like it). Thus, the exit value of the
               subprocess is really ("$? >> 8"), and "$? & 127" gives which
               signal, if any, the process died from, and "$? & 128" reports
               whether there was a core dump.

               Additionally, if the "h_errno" variable is supported in C, its
               value is returned via $? if any "gethost*()" function fails.

               If you have installed a signal handler for "SIGCHLD", the value
               of $? will usually be wrong outside that handler.

               Inside an "END" subroutine $? contains the value that is going
               to be given to "exit()". You can modify $? in an "END"
               subroutine to change the exit status of your program. For
               example:

                   END {
                       $? = 1 if $? == 255;  # die would make it 255
                   }

               Under VMS, the pragma "use vmsish 'status'" makes $? reflect
               the actual VMS exit status, instead of the default emulation of
               POSIX status; see "$?" in perlvms for details.

               Mnemonic: similar to sh and ksh.

       $EVAL_ERROR
       $@      The Perl syntax error message from the last "eval()" operator.
               If $@ is the null string, the last "eval()" parsed and executed
               correctly (although the operations you invoked may have failed
               in the normal fashion).

               Warning messages are not collected in this variable. You can,
               however, set up a routine to process warnings by setting
               $SIG{__WARN__} as described in "%SIG".

               Mnemonic: Where was the syntax error "at"?

   Deprecated and removed variables
       Deprecating a variable announces the intent of the perl maintainers to
       eventually remove the variable from the langauge. It may still be
       available despite its status. Using a deprecated variable triggers a
       warning.
               This is not the sigil you use in front of an array name to get
               the last index, like $#array. That's still how you get the last
               index of an array in Perl. The two have nothing to do with each
               other.

               Deprecated in Perl 5.

               Removed in Perl 5.10.

       $*      $* was a variable that you could use to enable multiline
               matching.  After a deprecation cycle, its magic was removed in
               Perl 5.10.  Using it now triggers a warning: "$* is no longer
               supported".  You should use the "/s" and "/m" regexp modifiers
               instead.

               Deprecated in Perl 5.

               Removed in Perl 5.10.

       $ARRAY_BASE
       $[      This variable stores the index of the first element in an
               array, and of the first character in a substring. The default
               is 0, but you could theoretically set it to 1 to make Perl
               behave more like awk (or Fortran) when subscripting and when
               evaluating the index() and substr() functions.

               As of release 5 of Perl, assignment to $[ is treated as a
               compiler directive, and cannot influence the behavior of any
               other file.  (That's why you can only assign compile-time
               constants to it.)  Its use is highly discouraged.

               Prior to Perl 5.10, assignment to $[ could be seen from outer
               lexical scopes in the same file, unlike other compile-time
               directives (such as strict). Using local() on it would bind its
               value strictly to a lexical block. Now it is always lexically
               scoped.

               Mnemonic: [ begins subscripts.

               Deprecated in Perl 5.12.

       $OLD_PERL_VERSION
       $]      See $^V for a more modern representation of the Perl version
               that allows accurate string comparisons.

               The version + patchlevel / 1000 of the Perl interpreter. This
               variable can be used to determine whether the Perl interpreter
               executing a script is in the right range of versions:

                   warn "No checksumming!\n" if $] < 3.019;

               The floating point representation can sometimes lead to
               inaccurate numeric comparisons.
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