perlrun


SYNOPSIS
       perl [ -sTtuUWX ]      [ -hv ] [ -V[:configvar] ]
            [ -cw ] [ -d[t][:debugger] ] [ -D[number/list] ]
            [ -pna ] [ -Fpattern ] [ -l[octal] ] [ -0[octal/hexadecimal] ]
            [ -Idir ] [ -m[-]module ] [ -M[-]'module...' ] [ -f ]
            [ -C [number/list] ]      [ -S ]      [ -x[dir] ]
            [ -i[extension] ]
            [ [-e|-E] 'command' ] [ -- ] [ programfile ] [ argument ]...

DESCRIPTION
       The normal way to run a Perl program is by making it directly
       executable, or else by passing the name of the source file as an
       argument on the command line.  (An interactive Perl environment is also
       possible--see perldebug for details on how to do that.)  Upon startup,
       Perl looks for your program in one of the following places:

       1.  Specified line by line via -e or -E switches on the command line.

       2.  Contained in the file specified by the first filename on the
           command line.  (Note that systems supporting the "#!" notation
           invoke interpreters this way. See "Location of Perl".)

       3.  Passed in implicitly via standard input.  This works only if there
           are no filename arguments--to pass arguments to a STDIN-read
           program you must explicitly specify a "-" for the program name.

       With methods 2 and 3, Perl starts parsing the input file from the
       beginning, unless you've specified a -x switch, in which case it scans
       for the first line starting with "#!" and containing the word "perl",
       and starts there instead.  This is useful for running a program
       embedded in a larger message.  (In this case you would indicate the end
       of the program using the "__END__" token.)

       The "#!" line is always examined for switches as the line is being
       parsed.  Thus, if you're on a machine that allows only one argument
       with the "#!" line, or worse, doesn't even recognize the "#!" line, you
       still can get consistent switch behaviour regardless of how Perl was
       invoked, even if -x was used to find the beginning of the program.

       Because historically some operating systems silently chopped off kernel
       interpretation of the "#!" line after 32 characters, some switches may
       be passed in on the command line, and some may not; you could even get
       a "-" without its letter, if you're not careful.  You probably want to
       make sure that all your switches fall either before or after that
       32-character boundary.  Most switches don't actually care if they're
       processed redundantly, but getting a "-" instead of a complete switch
       could cause Perl to try to execute standard input instead of your
       program.  And a partial -I switch could also cause odd results.

       Some switches do care if they are processed twice, for instance
       combinations of -l and -0.  Either put all the switches after the
       32-character boundary (if applicable), or replace the use of -0digits
       by "BEGIN{ $/ = "\0digits"; }".
       A similar trick involves the env program, if you have it.

           #!/usr/bin/env perl

       The examples above use a relative path to the perl interpreter, getting
       whatever version is first in the user's path.  If you want a specific
       version of Perl, say, perl5.14.1, you should place that directly in the
       "#!" line's path.

       If the "#!" line does not contain the word "perl" nor the word "indir"
       the program named after the "#!" is executed instead of the Perl
       interpreter.  This is slightly bizarre, but it helps people on machines
       that don't do "#!", because they can tell a program that their SHELL is
       /usr/bin/perl, and Perl will then dispatch the program to the correct
       interpreter for them.

       After locating your program, Perl compiles the entire program to an
       internal form.  If there are any compilation errors, execution of the
       program is not attempted.  (This is unlike the typical shell script,
       which might run part-way through before finding a syntax error.)

       If the program is syntactically correct, it is executed.  If the
       program runs off the end without hitting an exit() or die() operator,
       an implicit exit(0) is provided to indicate successful completion.

   #! and quoting on non-Unix systems
       Unix's "#!" technique can be simulated on other systems:

       OS/2
           Put

               extproc perl -S -your_switches

           as the first line in "*.cmd" file (-S due to a bug in cmd.exe's
           `extproc' handling).

       MS-DOS
           Create a batch file to run your program, and codify it in
           "ALTERNATE_SHEBANG" (see the dosish.h file in the source
           distribution for more information).

       Win95/NT
           The Win95/NT installation, when using the ActiveState installer for
           Perl, will modify the Registry to associate the .pl extension with
           the perl interpreter.  If you install Perl by other means
           (including building from the sources), you may have to modify the
           Registry yourself.  Note that this means you can no longer tell the
           difference between an executable Perl program and a Perl library
           file.

       VMS Put

            $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
            $ exit++ + ++$status != 0 and $exit = $status = undef;

       in your command-interpreter ("*", "\" and """ are common) and how to
       protect whitespace and these characters to run one-liners (see -e
       below).

       On some systems, you may have to change single-quotes to double ones,
       which you must not do on Unix or Plan 9 systems.  You might also have
       to change a single % to a %%.

       For example:

           # Unix
           perl -e 'print "Hello world\n"'

           # MS-DOS, etc.
           perl -e "print \"Hello world\n\""

           # VMS
           perl -e "print ""Hello world\n"""

       The problem is that none of this is reliable: it depends on the command
       and it is entirely possible neither works.  If 4DOS were the command
       shell, this would probably work better:

           perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""

       CMD.EXE in Windows NT slipped a lot of standard Unix functionality in
       when nobody was looking, but just try to find documentation for its
       quoting rules.

       There is no general solution to all of this.  It's just a mess.

   Location of Perl
       It may seem obvious to say, but Perl is useful only when users can
       easily find it.  When possible, it's good for both /usr/bin/perl and
       /usr/local/bin/perl to be symlinks to the actual binary.  If that can't
       be done, system administrators are strongly encouraged to put (symlinks
       to) perl and its accompanying utilities into a directory typically
       found along a user's PATH, or in some other obvious and convenient
       place.

       In this documentation, "#!/usr/bin/perl" on the first line of the
       program will stand in for whatever method works on your system.  You
       are advised to use a specific path if you care about a specific
       version.

           #!/usr/local/bin/perl5.14

       or if you just want to be running at least version, place a statement
       like this at the top of your program:

           use 5.014;

   Command Switches
       As with all standard commands, a single-character switch may be

                find . -name '*.orig' -print0 | perl -n0e unlink

            The special value 00 will cause Perl to slurp files in paragraph
            mode.  Any value 0400 or above will cause Perl to slurp files
            whole, but by convention the value 0777 is the one normally used
            for this purpose.

            You can also specify the separator character using hexadecimal
            notation: -0xHHH..., where the "H" are valid hexadecimal digits.
            Unlike the octal form, this one may be used to specify any Unicode
            character, even those beyond 0xFF.  So if you really want a record
            separator of 0777, specify it as -0x1FF.  (This means that you
            cannot use the -x option with a directory name that consists of
            hexadecimal digits, or else Perl will think you have specified a
            hex number to -0.)

       -a   turns on autosplit mode when used with a -n or -p.  An implicit
            split command to the @F array is done as the first thing inside
            the implicit while loop produced by the -n or -p.

                perl -ane 'print pop(@F), "\n";'

            is equivalent to

                while (<>) {
                    @F = split(' ');
                    print pop(@F), "\n";
                }

            An alternate delimiter may be specified using -F.

       -C [number/list]
            The -C flag controls some of the Perl Unicode features.

            As of 5.8.1, the -C can be followed either by a number or a list
            of option letters.  The letters, their numeric values, and effects
            are as follows; listing the letters is equal to summing the
            numbers.

                I     1   STDIN is assumed to be in UTF-8
                O     2   STDOUT will be in UTF-8
                E     4   STDERR will be in UTF-8
                S     7   I + O + E
                i     8   UTF-8 is the default PerlIO layer for input streams
                o    16   UTF-8 is the default PerlIO layer for output streams
                D    24   i + o
                A    32   the @ARGV elements are expected to be strings encoded
                          in UTF-8
                L    64   normally the "IOEioA" are unconditional, the L makes
                          them conditional on the locale environment variables
                          (the LC_ALL, LC_TYPE, and LANG, in the order of
                          decreasing precedence) -- if the variables indicate
                          UTF-8, then the selected "IOEioA" are in effect

            open() and with binmode() one can manipulate streams as usual.

            -C on its own (not followed by any number or option list), or the
            empty string "" for the "PERL_UNICODE" environment variable, has
            the same effect as -CSDL.  In other words, the standard I/O
            handles and the default "open()" layer are UTF-8-fied but only if
            the locale environment variables indicate a UTF-8 locale.  This
            behaviour follows the implicit (and problematic) UTF-8 behaviour
            of Perl 5.8.0.  (See "UTF-8 no longer default under UTF-8 locales"
            in perl581delta.)

            You can use -C0 (or "0" for "PERL_UNICODE") to explicitly disable
            all the above Unicode features.

            The read-only magic variable "${^UNICODE}" reflects the numeric
            value of this setting.  This variable is set during Perl startup
            and is thereafter read-only.  If you want runtime effects, use the
            three-arg open() (see "open" in perlfunc), the two-arg binmode()
            (see "binmode" in perlfunc), and the "open" pragma (see open).

            (In Perls earlier than 5.8.1 the -C switch was a Win32-only switch
            that enabled the use of Unicode-aware "wide system call" Win32
            APIs.  This feature was practically unused, however, and the
            command line switch was therefore "recycled".)

            Note: Since perl 5.10.1, if the -C option is used on the "#!"
            line, it must be specified on the command line as well, since the
            standard streams are already set up at this point in the execution
            of the perl interpreter.  You can also use binmode() to set the
            encoding of an I/O stream.

       -c   causes Perl to check the syntax of the program and then exit
            without executing it.  Actually, it will execute and "BEGIN",
            "UNITCHECK", or "CHECK" blocks and any "use" statements: these are
            considered as occurring outside the execution of your program.
            "INIT" and "END" blocks, however, will be skipped.

       -d
       -dt  runs the program under the Perl debugger.  See perldebug.  If t is
            specified, it indicates to the debugger that threads will be used
            in the code being debugged.

       -d:MOD[=bar,baz]
       -dt:MOD[=bar,baz]
            runs the program under the control of a debugging, profiling, or
            tracing module installed as "Devel::MOD". E.g., -d:DProf executes
            the program using the "Devel::DProf" profiler.  As with the -M
            flag, options may be passed to the "Devel::MOD" package where they
            will be received and interpreted by the "Devel::MOD::import"
            routine.  Again, like -M, use --d:-MOD to call
            "Devel::MOD::unimport" instead of import.  The comma-separated
            list of options must follow a "=" character.  If t is specified,
            it indicates to the debugger that threads will be used in the code
            being debugged.  See perldebug.
                    1  p  Tokenizing and parsing (with v, displays parse stack)
                    2  s  Stack snapshots (with v, displays all stacks)
                    4  l  Context (loop) stack processing
                    8  t  Trace execution
                   16  o  Method and overloading resolution
                   32  c  String/numeric conversions
                   64  P  Print profiling info, source file input state
                  128  m  Memory and SV allocation
                  256  f  Format processing
                  512  r  Regular expression parsing and execution
                 1024  x  Syntax tree dump
                 2048  u  Tainting checks
                 4096  U  Unofficial, User hacking (reserved for private,
                          unreleased use)
                 8192  H  Hash dump -- usurps values()
                16384  X  Scratchpad allocation
                32768  D  Cleaning up
                65536  S  Op slab allocation
               131072  T  Tokenizing
               262144  R  Include reference counts of dumped variables (eg when
                          using -Ds)
               524288  J  show s,t,P-debug (don't Jump over) on opcodes within
                          package DB
              1048576  v  Verbose: use in conjunction with other flags
              2097152  C  Copy On Write
              4194304  A  Consistency checks on internal structures
              8388608  q  quiet - currently only suppresses the "EXECUTING"
                          message
             16777216  M  trace smart match resolution
             33554432  B  dump suBroutine definitions, including special Blocks
                          like BEGIN

            All these flags require -DDEBUGGING when you compile the Perl
            executable (but see ":opd" in Devel::Peek or "'debug' mode" in re
            which may change this).  See the INSTALL file in the Perl source
            distribution for how to do this.  This flag is automatically set
            if you include -g option when "Configure" asks you about
            optimizer/debugger flags.

            If you're just trying to get a print out of each line of Perl code
            as it executes, the way that "sh -x" provides for shell scripts,
            you can't use Perl's -D switch.  Instead do this

              # If you have "env" utility
              env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program

              # Bourne shell syntax
              $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program

              # csh syntax
              % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)

            See perldebug for details and variations.

            Perl can be built so that it by default will try to execute
            $Config{sitelib}/sitecustomize.pl at startup (in a BEGIN block).
            This is a hook that allows the sysadmin to customize how Perl
            behaves.  It can for instance be used to add entries to the @INC
            array to make Perl find modules in non-standard locations.

            Perl actually inserts the following code:

                BEGIN {
                    do { local $!; -f "$Config{sitelib}/sitecustomize.pl"; }
                        && do "$Config{sitelib}/sitecustomize.pl";
                }

            Since it is an actual "do" (not a "require"), sitecustomize.pl
            doesn't need to return a true value. The code is run in package
            "main", in its own lexical scope. However, if the script dies, $@
            will not be set.

            The value of $Config{sitelib} is also determined in C code and not
            read from "Config.pm", which is not loaded.

            The code is executed very early. For example, any changes made to
            @INC will show up in the output of `perl -V`. Of course, "END"
            blocks will be likewise executed very late.

            To determine at runtime if this capability has been compiled in
            your perl, you can check the value of $Config{usesitecustomize}.

       -Fpattern
            specifies the pattern to split on if -a is also in effect.  The
            pattern may be surrounded by "//", "", or '', otherwise it will be
            put in single quotes. You can't use literal whitespace in the
            pattern.

       -h   prints a summary of the options.

       -i[extension]
            specifies that files processed by the "<>" construct are to be
            edited in-place.  It does this by renaming the input file, opening
            the output file by the original name, and selecting that output
            file as the default for print() statements.  The extension, if
            supplied, is used to modify the name of the old file to make a
            backup copy, following these rules:

            If no extension is supplied, and your system supports it, the
            original file is kept open without a name while the output is
            redirected to a new file with the original filename.  When perl
            exits, cleanly or not, the original file is unlinked.

            If the extension doesn't contain a "*", then it is appended to the
            end of the current filename as a suffix.  If the extension does
            contain one or more "*" characters, then each "*" is replaced with
            the current filename.  In Perl terms, you could think of this as:

                                                           # 'old/fileA.orig'

            These sets of one-liners are equivalent:

             $ perl -pi -e 's/bar/baz/' fileA          # overwrite current file
             $ perl -pi'*' -e 's/bar/baz/' fileA       # overwrite current file

             $ perl -pi'.orig' -e 's/bar/baz/' fileA   # backup to 'fileA.orig'
             $ perl -pi'*.orig' -e 's/bar/baz/' fileA  # backup to 'fileA.orig'

            From the shell, saying

                $ perl -p -i.orig -e "s/foo/bar/; ... "

            is the same as using the program:

                #!/usr/bin/perl -pi.orig
                s/foo/bar/;

            which is equivalent to

                #!/usr/bin/perl
                $extension = '.orig';
                LINE: while (<>) {
                    if ($ARGV ne $oldargv) {
                        if ($extension !~ /\*/) {
                            $backup = $ARGV . $extension;
                        }
                        else {
                            ($backup = $extension) =~ s/\*/$ARGV/g;
                        }
                        rename($ARGV, $backup);
                        open(ARGVOUT, ">$ARGV");
                        select(ARGVOUT);
                        $oldargv = $ARGV;
                    }
                    s/foo/bar/;
                }
                continue {
                    print;  # this prints to original filename
                }
                select(STDOUT);

            except that the -i form doesn't need to compare $ARGV to $oldargv
            to know when the filename has changed.  It does, however, use
            ARGVOUT for the selected filehandle.  Note that STDOUT is restored
            as the default output filehandle after the loop.

            As shown above, Perl creates the backup file whether or not any
            output is actually changed.  So this is just a fancy way to copy
            files:

                $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
            or
            clobber protected files?  Isn't this a bug in Perl?" in perlfaq5.

            You cannot use -i to create directories or to strip extensions
            from files.

            Perl does not expand "~" in filenames, which is good, since some
            folks use it for their backup files:

                $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...

            Note that because -i renames or deletes the original file before
            creating a new file of the same name, Unix-style soft and hard
            links will not be preserved.

            Finally, the -i switch does not impede execution when no files are
            given on the command line.  In this case, no backup is made (the
            original file cannot, of course, be determined) and processing
            proceeds from STDIN to STDOUT as might be expected.

       -Idirectory
            Directories specified by -I are prepended to the search path for
            modules (@INC).

       -l[octnum]
            enables automatic line-ending processing.  It has two separate
            effects.  First, it automatically chomps $/ (the input record
            separator) when used with -n or -p.  Second, it assigns "$\" (the
            output record separator) to have the value of octnum so that any
            print statements will have that separator added back on.  If
            octnum is omitted, sets "$\" to the current value of $/.  For
            instance, to trim lines to 80 columns:

                perl -lpe 'substr($_, 80) = ""'

            Note that the assignment "$\ = $/" is done when the switch is
            processed, so the input record separator can be different than the
            output record separator if the -l switch is followed by a -0
            switch:

                gnufind / -print0 | perl -ln0e 'print "found $_" if -p'

            This sets "$\" to newline and then sets $/ to the null character.

       -m[-]module
       -M[-]module
       -M[-]'module ...'
       -[mM][-]module=arg[,arg]...
            -mmodule executes "use" module "();" before executing your
            program.

            -Mmodule executes "use" module ";" before executing your program.
            You can use quotes to add extra code after the module name, e.g.,
            '-MMODULE qw(foo bar)'.

            check, which could happen for example if MODULE inherits from
            Exporter.

       -n   causes Perl to assume the following loop around your program,
            which makes it iterate over filename arguments somewhat like sed
            -n or awk:

              LINE:
                while (<>) {
                    ...             # your program goes here
                }

            Note that the lines are not printed by default.  See "-p" to have
            lines printed.  If a file named by an argument cannot be opened
            for some reason, Perl warns you about it and moves on to the next
            file.

            Also note that "<>" passes command line arguments to "open" in
            perlfunc, which doesn't necessarily interpret them as file names.
            See  perlop for possible security implications.

            Here is an efficient way to delete all files that haven't been
            modified for at least a week:

                find . -mtime +7 -print | perl -nle unlink

            This is faster than using the -exec switch of find because you
            don't have to start a process on every filename found.  It does
            suffer from the bug of mishandling newlines in pathnames, which
            you can fix if you follow the example under -0.

            "BEGIN" and "END" blocks may be used to capture control before or
            after the implicit program loop, just as in awk.

       -p   causes Perl to assume the following loop around your program,
            which makes it iterate over filename arguments somewhat like sed:

              LINE:
                while (<>) {
                    ...             # your program goes here
                } continue {
                    print or die "-p destination: $!\n";
                }

            If a file named by an argument cannot be opened for some reason,
            Perl warns you about it, and moves on to the next file.  Note that
            the lines are printed automatically.  An error occurring during
            printing is treated as fatal.  To suppress printing use the -n
            switch.  A -p overrides a -n switch.

            "BEGIN" and "END" blocks may be used to capture control before or
            after the implicit loop, just as in awk.

       -s   enables rudimentary switch parsing for switches on the command
            spurious "used only once" warnings.

       -S   makes Perl use the PATH environment variable to search for the
            program unless the name of the program contains path separators.

            On some platforms, this also makes Perl append suffixes to the
            filename while searching for it.  For example, on Win32 platforms,
            the ".bat" and ".cmd" suffixes are appended if a lookup for the
            original name fails, and if the name does not already end in one
            of those suffixes.  If your Perl was compiled with "DEBUGGING"
            turned on, using the -Dp switch to Perl shows how the search
            progresses.

            Typically this is used to emulate "#!" startup on platforms that
            don't support "#!".  It's also convenient when debugging a script
            that uses "#!", and is thus normally found by the shell's $PATH
            search mechanism.

            This example works on many platforms that have a shell compatible
            with Bourne shell:

                #!/usr/bin/perl
                eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
                        if $running_under_some_shell;

            The system ignores the first line and feeds the program to
            /bin/sh, which proceeds to try to execute the Perl program as a
            shell script.  The shell executes the second line as a normal
            shell command, and thus starts up the Perl interpreter.  On some
            systems $0 doesn't always contain the full pathname, so the -S
            tells Perl to search for the program if necessary.  After Perl
            locates the program, it parses the lines and ignores them because
            the variable $running_under_some_shell is never true.  If the
            program will be interpreted by csh, you will need to replace
            "${1+"$@"}" with $*, even though that doesn't understand embedded
            spaces (and such) in the argument list.  To start up sh rather
            than csh, some systems may have to replace the "#!" line with a
            line containing just a colon, which will be politely ignored by
            Perl.  Other systems can't control that, and need a totally
            devious construct that will work under any of csh, sh, or Perl,
            such as the following:

                    eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
                    & eval 'exec /usr/bin/perl -wS $0 $argv:q'
                            if $running_under_some_shell;

            If the filename supplied contains directory separators (and so is
            an absolute or relative pathname), and if that file is not found,
            platforms that append file extensions will do so and try to look
            for the file with those extensions added, one by one.

            On DOS-like platforms, if the program does not contain directory
            separators, it will first be searched for in the current directory
            before being searched for on the PATH.  On Unix platforms, the
            are done only when running setuid or setgid.  It's a good idea to
            turn them on explicitly for programs that run on behalf of someone
            else whom you might not necessarily trust, such as CGI programs or
            any internet servers you might write in Perl.  See perlsec for
            details.  For security reasons, this option must be seen by Perl
            quite early; usually this means it must appear early on the
            command line or in the "#!" line for systems which support that
            construct.

       -u   This switch causes Perl to dump core after compiling your program.
            You can then in theory take this core dump and turn it into an
            executable file by using the undump program (not supplied).  This
            speeds startup at the expense of some disk space (which you can
            minimize by stripping the executable).  (Still, a "hello world"
            executable comes out to about 200K on my machine.)  If you want to
            execute a portion of your program before dumping, use the dump()
            operator instead.  Note: availability of undump is platform
            specific and may not be available for a specific port of Perl.

       -U   allows Perl to do unsafe operations.  Currently the only "unsafe"
            operations are attempting to unlink directories while running as
            superuser and running setuid programs with fatal taint checks
            turned into warnings.  Note that warnings must be enabled along
            with this option to actually generate the taint-check warnings.

       -v   prints the version and patchlevel of your perl executable.

       -V   prints summary of the major perl configuration values and the
            current values of @INC.

       -V:configvar
            Prints to STDOUT the value of the named configuration variable(s),
            with multiples when your "configvar" argument looks like a regex
            (has non-letters).  For example:

                $ perl -V:libc
                    libc='/lib/libc-2.2.4.so';
                $ perl -V:lib.
                    libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
                    libc='/lib/libc-2.2.4.so';
                $ perl -V:lib.*
                    libpth='/usr/local/lib /lib /usr/lib';
                    libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
                    lib_ext='.a';
                    libc='/lib/libc-2.2.4.so';
                    libperl='libperl.a';
                    ....

            Additionally, extra colons can be used to control formatting.  A
            trailing colon suppresses the linefeed and terminator ";",
            allowing you to embed queries into shell commands.  (mnemonic:
            PATH separator ":".)

                $ echo "compression-vars: " `perl -V:z.*: ` " are here !"

                $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
                building_on 'linux' '5' '1' '9' now

       -w   prints warnings about dubious constructs, such as variable names
            mentioned only once and scalar variables used before being set;
            redefined subroutines; references to undefined filehandles;
            filehandles opened read-only that you are attempting to write on;
            values used as a number that don't look like numbers; using an
            array as though it were a scalar; if your subroutines recurse more
            than 100 deep; and innumerable other things.

            This switch really just enables the global $^W variable; normally,
            the lexically scoped "use warnings" pragma is preferred. You can
            disable or promote into fatal errors specific warnings using
            "__WARN__" hooks, as described in perlvar and "warn" in perlfunc.
            See also perldiag and perltrap.  A fine-grained warning facility
            is also available if you want to manipulate entire classes of
            warnings; see warnings or perllexwarn.

       -W   Enables all warnings regardless of "no warnings" or $^W.  See
            perllexwarn.

       -X   Disables all warnings regardless of "use warnings" or $^W.  See
            perllexwarn.

       -x
       -xdirectory
            tells Perl that the program is embedded in a larger chunk of
            unrelated text, such as in a mail message.  Leading garbage will
            be discarded until the first line that starts with "#!" and
            contains the string "perl".  Any meaningful switches on that line
            will be applied.

            All references to line numbers by the program (warnings, errors,
            ...)  will treat the "#!" line as the first line.  Thus a warning
            on the 2nd line of the program, which is on the 100th line in the
            file will be reported as line 2, not as line 100.  This can be
            overridden by using the "#line" directive.  (See "Plain Old
            Comments (Not!)" in perlsyn)

            If a directory name is specified, Perl will switch to that
            directory before running the program.  The -x switch controls only
            the disposal of leading garbage.  The program must be terminated
            with "__END__" if there is trailing garbage to be ignored;  the
            program can process any or all of the trailing garbage via the
            "DATA" filehandle if desired.

            The directory, if specified, must appear immediately following the
            -x with no intervening whitespace.

ENVIRONMENT
       HOME        Used if "chdir" has no argument.

                   matching the entries in $Config{inc_version_list} are
                   added.  (These typically would be for older compatible perl
                   versions installed in the same directory tree.)

                   If PERL5LIB is not defined, PERLLIB is used.  Directories
                   are separated (like in PATH) by a colon on Unixish
                   platforms and by a semicolon on Windows (the proper path
                   separator being given by the command "perl -V:path_sep").

                   When running taint checks, either because the program was
                   running setuid or setgid, or the -T or -t switch was
                   specified, neither PERL5LIB nor PERLLIB is consulted. The
                   program should instead say:

                       use lib "/my/directory";

       PERL5OPT    Command-line options (switches).  Switches in this variable
                   are treated as if they were on every Perl command line.
                   Only the -[CDIMUdmtwW] switches are allowed.  When running
                   taint checks (either because the program was running setuid
                   or setgid, or because the -T or -t switch was used), this
                   variable is ignored.  If PERL5OPT begins with -T, tainting
                   will be enabled and subsequent options ignored.  If
                   PERL5OPT begins with -t, tainting will be enabled, a
                   writable dot removed from @INC, and subsequent options
                   honored.

       PERLIO      A space (or colon) separated list of PerlIO layers. If perl
                   is built to use PerlIO system for IO (the default) these
                   layers affect Perl's IO.

                   It is conventional to start layer names with a colon (for
                   example, ":perlio") to emphasize their similarity to
                   variable "attributes". But the code that parses layer
                   specification strings, which is also used to decode the
                   PERLIO environment variable, treats the colon as a
                   separator.

                   An unset or empty PERLIO is equivalent to the default set
                   of layers for your platform; for example, ":unix:perlio" on
                   Unix-like systems and ":unix:crlf" on Windows and other
                   DOS-like systems.

                   The list becomes the default for all Perl's IO.
                   Consequently only built-in layers can appear in this list,
                   as external layers (such as ":encoding()") need IO in order
                   to load them!  See "open pragma" for how to add external
                   encodings as defaults.

                   Layers it makes sense to include in the PERLIO environment
                   variable are briefly summarized below. For more details see
                   PerlIO.

                   :bytes  A pseudolayer that turns the ":utf8" flag off for
                   :mmap   A layer that implements "reading" of files by using
                           mmap(2) to make an entire file appear in the
                           process's address space, and then using that as
                           PerlIO's "buffer".

                   :perlio This is a re-implementation of stdio-like buffering
                           written as a PerlIO layer.  As such it will call
                           whatever layer is below it for its operations,
                           typically ":unix".

                   :pop    An experimental pseudolayer that removes the
                           topmost layer.  Use with the same care as is
                           reserved for nitroglycerine.

                   :raw    A pseudolayer that manipulates other layers.
                           Applying the ":raw" layer is equivalent to calling
                           "binmode($fh)".  It makes the stream pass each byte
                           as-is without translation.  In particular, both
                           CRLF translation and intuiting ":utf8" from the
                           locale are disabled.

                           Unlike in earlier versions of Perl, ":raw" is not
                           just the inverse of ":crlf": other layers which
                           would affect the binary nature of the stream are
                           also removed or disabled.

                   :stdio  This layer provides a PerlIO interface by wrapping
                           system's ANSI C "stdio" library calls. The layer
                           provides both buffering and IO.  Note that the
                           ":stdio" layer does not do CRLF translation even if
                           that is the platform's normal behaviour. You will
                           need a ":crlf" layer above it to do that.

                   :unix   Low-level layer that calls "read", "write",
                           "lseek", etc.

                   :utf8   A pseudolayer that enables a flag in the layer
                           below to tell Perl that output should be in utf8
                           and that input should be regarded as already in
                           valid utf8 form. WARNING: It does not check for
                           validity and as such should be handled with extreme
                           caution for input, because security violations can
                           occur with non-shortest UTF-8 encodings, etc.
                           Generally ":encoding(utf8)" is the best option when
                           reading UTF-8 encoded data.

                   :win32  On Win32 platforms this experimental layer uses
                           native "handle" IO rather than a Unix-like numeric
                           file descriptor layer. Known to be buggy in this
                           release (5.14).

                   The default set of layers should give acceptable results on
                   all platforms


                   This release (5.14) uses "unix" as the bottom layer on
                   Win32, and so still uses the C compiler's numeric file
                   descriptor routines. There is an experimental native
                   "win32" layer, which is expected to be enhanced and should
                   eventually become the default under Win32.

                   The PERLIO environment variable is completely ignored when
                   Perl is run in taint mode.

       PERLIO_DEBUG
                   If set to the name of a file or device, certain operations
                   of PerlIO subsystem will be logged to that file, which is
                   opened in append mode.  Typical uses are in Unix:

                      % env PERLIO_DEBUG=/dev/tty perl script ...

                   and under Win32, the approximately equivalent:

                      > set PERLIO_DEBUG=CON
                      perl script ...

                   This functionality is disabled for setuid scripts and for
                   scripts run with -T.

       PERLLIB     A list of directories in which to look for Perl library
                   files before looking in the standard library and the
                   current directory.  If PERL5LIB is defined, PERLLIB is not
                   used.

                   The PERLLIB environment variable is completely ignored when
                   Perl is run in taint mode.

       PERL5DB     The command used to load the debugger code.  The default
                   is:

                           BEGIN { require "perl5db.pl" }

                   The PERL5DB environment variable is only used when Perl is
                   started with a bare -d switch.

       PERL5DB_THREADED
                   If set to a true value, indicates to the debugger that the
                   code being debugged uses threads.

       PERL5SHELL (specific to the Win32 port)
                   On Win32 ports only, may be set to an alternative shell
                   that Perl must use internally for executing "backtick"
                   commands or system().  Default is "cmd.exe /x/d/c" on
                   WindowsNT and "command.com /c" on Windows95.  The value is
                   considered space-separated.  Precede any character that
                   needs to be protected, like a space or backslash, with
                   another backslash.


       PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
                   Set to 1 to allow the use of non-IFS compatible LSPs
                   (Layered Service Providers).  Perl normally searches for an
                   IFS-compatible LSP because this is required for its
                   emulation of Windows sockets as real filehandles.  However,
                   this may cause problems if you have a firewall such as
                   McAfee Guardian, which requires that all applications use
                   its LSP but which is not IFS-compatible, because clearly
                   Perl will normally avoid using such an LSP.

                   Setting this environment variable to 1 means that Perl will
                   simply use the first suitable LSP enumerated in the
                   catalog, which keeps McAfee Guardian happy--and in that
                   particular case Perl still works too because McAfee
                   Guardian's LSP actually plays other games which allow
                   applications requiring IFS compatibility to work.

       PERL_DEBUG_MSTATS
                   Relevant only if Perl is compiled with the "malloc"
                   included with the Perl distribution; that is, if "perl
                   -V:d_mymalloc" is "define".

                   If set, this dumps out memory statistics after execution.
                   If set to an integer greater than one, also dumps out
                   memory statistics after compilation.

       PERL_DESTRUCT_LEVEL
                   Relevant only if your Perl executable was built with
                   -DDEBUGGING, this controls the behaviour of global
                   destruction of objects and other references.  See
                   "PERL_DESTRUCT_LEVEL" in perlhacktips for more information.

       PERL_DL_NONLAZY
                   Set to "1" to have Perl resolve all undefined symbols when
                   it loads a dynamic library.  The default behaviour is to
                   resolve symbols when they are used.  Setting this variable
                   is useful during testing of extensions, as it ensures that
                   you get an error on misspelled function names even if the
                   test suite doesn't call them.

       PERL_ENCODING
                   If using the "use encoding" pragma without an explicit
                   encoding name, the PERL_ENCODING environment variable is
                   consulted for an encoding name.

       PERL_HASH_SEED
                   (Since Perl 5.8.1, new semantics in Perl 5.18.0)  Used to
                   override the randomization of Perl's internal hash
                   function. The value is expressed in hexadecimal, and may
                   include a leading 0x. Truncated patterns are treated as
                   though they are suffixed with sufficient 0's as required.

                   If the option is provided, and "PERL_PERTURB_KEYS" is NOT

       PERL_PERTURB_KEYS
                   (Since Perl 5.18.0)  Set to "0" or "NO" then traversing
                   keys will be repeatable from run to run for the same
                   PERL_HASH_SEED.  Insertion into a hash will not change the
                   order, except to provide for more space in the hash. When
                   combined with setting PERL_HASH_SEED this mode is as close
                   to pre 5.18 behavior as you can get.

                   When set to "1" or "RANDOM" then traversing keys will be
                   randomized.  Every time a hash is inserted into the key
                   order will change in a random fashion. The order may not be
                   repeatable in a following program run even if the
                   PERL_HASH_SEED has been specified. This is the default mode
                   for perl.

                   When set to "2" or "DETERMINISTIC" then inserting keys into
                   a hash will cause the key order to change, but in a way
                   that is repeatable from program run to program run.

                   NOTE: Use of this option is considered insecure, and is
                   intended only for debugging non-deterministic behavior in
                   Perl's hash function. Do not use it in production.

                   See "Algorithmic Complexity Attacks" in perlsec and
                   "PERL_HASH_SEED" and "PERL_HASH_SEED_DEBUG" for more
                   information. You can get and set the key traversal mask for
                   a specific hash by using the "hash_traversal_mask()"
                   function from Hash::Util.

       PERL_HASH_SEED_DEBUG
                   (Since Perl 5.8.1.)  Set to "1" to display (to STDERR)
                   information about the hash function, seed, and what type of
                   key traversal randomization is in effect at the beginning
                   of execution.  This, combined with "PERL_HASH_SEED" and
                   "PERL_PERTURB_KEYS" is intended to aid in debugging
                   nondeterministic behaviour caused by hash randomization.

                   Note that any information about the hash function,
                   especially the hash seed is sensitive information: by
                   knowing it, one can craft a denial-of-service attack
                   against Perl code, even remotely; see "Algorithmic
                   Complexity Attacks" in perlsec for more information. Do not
                   disclose the hash seed to people who don't need to know it.
                   See also "hash_seed()" and "key_traversal_mask()" in
                   Hash::Util.

                   An example output might be:

                       HASH_FUNCTION = ONE_AT_A_TIME_HARD HASH_SEED = 0x652e9b9349a7a032 PERTURB_KEYS = 1 (RANDOM)

       PERL_MEM_LOG
                   If your Perl was configured with -Accflags=-DPERL_MEM_LOG,
                   setting the environment variable "PERL_MEM_LOG" enables
                   logging debug messages. The value has the form
                   Perl and the logical device for the @INC path on VMS only.
                   Other logical names that affect Perl on VMS include
                   PERLSHR, PERL_ENV_TABLES, and SYS$TIMEZONE_DIFFERENTIAL,
                   but are optional and discussed further in perlvms and in
                   README.vms in the Perl source distribution.

       PERL_SIGNALS
                   Available in Perls 5.8.1 and later.  If set to "unsafe",
                   the pre-Perl-5.8.0 signal behaviour (which is immediate but
                   unsafe) is restored.  If set to "safe", then safe (but
                   deferred) signals are used.  See "Deferred Signals (Safe
                   Signals)" in perlipc.

       PERL_UNICODE
                   Equivalent to the -C command-line switch.  Note that this
                   is not a boolean variable. Setting this to "1" is not the
                   right way to "enable Unicode" (whatever that would mean).
                   You can use "0" to "disable Unicode", though (or
                   alternatively unset PERL_UNICODE in your shell before
                   starting Perl).  See the description of the -C switch for
                   more information.

       SYS$LOGIN (specific to the VMS port)
                   Used if chdir has no argument and HOME and LOGDIR are not
                   set.

       Perl also has environment variables that control how Perl handles data
       specific to particular natural languages; see perllocale.

       Perl and its various modules and components, including its test
       frameworks, may sometimes make use of certain other environment
       variables.  Some of these are specific to a particular platform.
       Please consult the appropriate module documentation and any
       documentation for your platform (like perlsolaris, perllinux,
       perlmacosx, perlwin32, etc) for variables peculiar to those specific
       situations.

       Perl makes all environment variables available to the program being
       executed, and passes these along to any child processes it starts.
       However, programs running setuid would do well to execute the following
       lines before doing anything else, just to keep people honest:

           $ENV{PATH}  = "/bin:/usr/bin";    # or whatever you need
           $ENV{SHELL} = "/bin/sh" if exists $ENV{SHELL};
           delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};



perl v5.18.2                      2014-01-06                        PERLRUN(1)
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