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perlrun (1)


perlrun - how to execute the Perl interpreter


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 ]...


Perl Programmers Reference Guide                       PERLRUN(1)

     perlrun - how to execute the Perl interpreter

     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 ]...

     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

     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
     behavior 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

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     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"; }".

     Parsing of the #! switches starts wherever "perl" is
     mentioned in the line.  The sequences "-*" and "- " are
     specifically ignored so that you could, if you were so
     inclined, say

         #! -*-perl-*-
         eval 'exec perl -x -wS $0 ${1+"$@"}'
             if 0;

     to let Perl see the -p switch.

     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,
     perl5.005_57, you should place that directly in the #!
     line's path.

     If the #! line does not contain the word "perl", 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

     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

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     successful completion.

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


             extproc perl -S -your_switches

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

         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).

         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;

         at the top of your program, where -mysw are any command
         line switches you want to pass to Perl.  You can now
         invoke the program directly, by saying "perl program",
         or as a DCL procedure, by saying @program (or implicitly
         via DCL$PATH by just using the name of the program).

         This incantation is a bit much to remember, but Perl
         will display it for you if you say "perl

     Command-interpreters on non-Unix systems have rather
     different ideas on quoting than Unix shells.  You'll need to
     learn the special characters 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 %%.

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     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

         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

  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

     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.


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

         use 5.005_54;

  Command Switches
     As with all standard commands, a single-character switch may
     be clustered with the following switch, if any.

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         #!/usr/bin/perl -spi.orig   # same as -s -p -i.orig

     Switches include:

          specifies the input record separator ($/) as an octal
          or hexadecimal number.  If there are no digits, the
          null character is the separator.  Other switches may
          precede or follow the digits.  For example, if you have
          a version of find which can print filenames terminated
          by the null character, you can say this:

              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.  (This means that you cannot use the
          "-x" with a directory name that consists of hexadecimal

     -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

          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.

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              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
              a   256   Set ${^UTF8CACHE} to -1, to run the UTF-8 caching code in
                        debugging mode.

          For example, "-COE" and "-C6" will both turn on
          UTF-8-ness on both STDOUT and STDERR.  Repeating
          letters is just redundant, not cumulative nor toggling.

          The "io" options mean that any subsequent open() (or
          similar I/O operations) will have the ":utf8" PerlIO
          layer implicitly applied to them, in other words, UTF-8
          is expected from any input stream, and UTF-8 is
          produced to any output stream.  This is just the
          default, with explicit layers in 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.

          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

          (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

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          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
          "BEGIN", "UNITCHECK", "CHECK", and "use" blocks,
          because these are considered as occurring outside the
          execution of your program.  "INIT" and "END" blocks,
          however, will be skipped.

     -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

          runs the program under the control of a debugging,
          profiling, or tracing module installed as Devel::foo.
          E.g., -d:DProf executes the program using the
          Devel::DProf profiler.  As with the -M flag, options
          may be passed to the Devel::foo package where they will
          be received and interpreted by the Devel::foo::import
          routine.  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.

          sets debugging flags.  To watch how it executes your
          program, use -Dtls.  (This works only if debugging is
          compiled into your Perl.)  Another nice value is -Dx,
          which lists your compiled syntax tree.  And -Dr
          displays compiled regular expressions; the format of
          the output is explained in perldebguts.

          As an alternative, specify a number instead of list of
          letters (e.g., -D14 is equivalent to -Dtls):

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                  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
             131072  T  Tokenising
             262144  R  Include reference counts of dumped variables (eg when using -Ds)
             524288  J  Do not s,t,P-debug (Jump over) 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 Devel::Peek, 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.

     -e commandline
          may be used to enter one line of program.  If -e is
          given, Perl will not look for a filename in the

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          argument list.  Multiple -e commands may be given to
          build up a multi-line script.  Make sure to use
          semicolons where you would in a normal program.

     -E commandline
          behaves just like -e, except that it implicitly enables
          all optional features (in the main compilation unit).
          See feature.

     -f   Disable executing $Config{sitelib}/ at

          Perl can be built so that it by default will try to
          execute $Config{sitelib}/ 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}/"; }
                      && do "$Config{sitelib}/";

          Since it is an actual "do" (not a "require"),
 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 "", which is not

          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

          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.

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          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, no backup is made and the
          current file is overwritten.

          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:

              ($backup = $extension) =~ s/\*/$file_name/g;

          This allows you to add a prefix to the backup file,
          instead of (or in addition to) a suffix:

              $ perl -pi'orig_*' -e 's/bar/baz/' fileA    # backup to 'orig_fileA'

          Or even to place backup copies of the original files
          into another directory (provided the directory already

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

          which is equivalent to

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

          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...
              $ perl -p -i'.orig' -e 1 file1 file2 file3...

          You can use "eof" without parentheses to locate the end
          of each input file, in case you want to append to each
          file, or reset line numbering (see example in "eof" in

          If, for a given file, Perl is unable to create the
          backup file as specified in the extension then it will
          skip that file and continue on with the next one (if it

          For a discussion of issues surrounding file permissions
          and -i, see "Why does Perl let me delete read-only
          files?  Why does -i clobber protected files?  Isn't
          this a bug in Perl?" in perlfaq5.

          You cannot use -i to create directories or to strip

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          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.

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

          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

     -M[-]'module ...'
          -mmodule executes "use" module "();" before executing
          your program.

          -Mmodule executes "use" module ";" before executing
          your program.  You can use quotes to add extra code

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          after the module name, e.g., '-Mmodule qw(foo bar)'.

          If the first character after the -M or -m is a dash
          ("-") then the 'use' is replaced with 'no'.

          A little builtin syntactic sugar means you can also say
          -mmodule=foo,bar or -Mmodule=foo,bar as a shortcut for
          '-Mmodule qw(foo bar)'.  This avoids the need to use
          quotes when importing symbols.  The actual code
          generated by -Mmodule=foo,bar is "use module
          split(/,/,q{foo,bar})".  Note that the "=" form removes
          the distinction between -m and -M.

          A consequence of this is that -MFoo=number never does a
          version check (unless "Foo::import()" itself is set up
          to do a version check, which could happen for example
          if Foo 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:

              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

          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

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

              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 line after the program name but before any
          filename arguments (or before an argument of --).  Any
          switch found there is removed from @ARGV and sets the
          corresponding variable in the Perl program.  The
          following program prints "1" if the program is invoked
          with a -xyz switch, and "abc" if it is invoked with

              #!/usr/bin/perl -s
              if ($xyz) { print "$xyz\n" }

          Do note that a switch like --help creates the variable
          ${-help}, which is not compliant with "strict refs".
          Also, when using this option on a script with warnings
          enabled you may get a lot of spurious "used only once"

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

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          Typically this is used to emulate #! startup on
          platforms that don't support #!.  Its 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:

              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
          (i.e., 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 program will be searched
          for strictly on the PATH.

     -t   Like -T, but taint checks will issue warnings rather
          than fatal errors.  These warnings can be controlled
          normally with "no warnings qw(taint)".

          NOTE: this is not a substitute for -T. This is meant

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          only to be used as a temporary development aid while
          securing legacy code: for real production code and for
          new secure code written from scratch always use the
          real -T.

     -T   forces "taint" checks to be turned on so you can test
          them.  Ordinarily these checks 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

     -u   This obsolete 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 the -w switch (or the $^W
          variable) must be used along with this option to
          actually generate the taint-check warnings.

     -v   prints the version and patchlevel of your perl

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

          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

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

          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 !"
              compression-vars:  zcat='' zip='zip'  are here !

          A leading colon removes the 'name=' part of the
          response, this allows you to map to the name you need.
          (mnemonic: empty label)

              $ echo "goodvfork="`./perl -Ilib -V::usevfork`

          Leading and trailing colons can be used together if you
          need positional parameter values without the names.
          Note that in the case below, the PERL_API params are
          returned in alphabetical order.

              $ 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 that are mentioned only once and scalar
          variables that are used before being set, redefined
          subroutines, references to undefined filehandles or
          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 internal $^W
          variable.  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 new, fine-grained warning facility is
          also available if you want to manipulate entire classes

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          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.

          tells Perl that the program is embedded in a larger
          chunk of unrelated ASCII 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

          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, and 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.

     HOME        Used if chdir has no argument.

     LOGDIR      Used if chdir has no argument and HOME is not

     PATH        Used in executing subprocesses, and in finding
                 the program if -S is used.

     PERL5LIB    A list of directories in which to look for Perl
                 library files before looking in the standard
                 library and the current directory.  Any
                 architecture-specific directories under the
                 specified locations are automatically included
                 if they exist (this lookup being done at
                 interpreter startup time.)

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                 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 variable is
                 used. The program should instead say:

                     use lib "/my/directory";

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

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

                 It is conventional to start layer names with a
                 colon e.g. ":perlio" to emphasise 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

                 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

                 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.

                 The layers that it makes sense to include in the
                 PERLIO environment variable are briefly
                 summarised below. For more details see PerlIO.

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                 :bytes  A pseudolayer that turns off the ":utf8"
                         flag for the layer below.  Unlikely to
                         be useful on its own in the global
                         PERLIO environment variable.  You
                         perhaps were thinking of ":crlf:bytes"
                         or ":perlio:bytes".

                 :crlf   A layer which does CRLF to "\n"
                         translation distinguishing "text" and
                         "binary" files in the manner of MS-DOS
                         and similar operating systems.  (It
                         currently does not mimic MS-DOS as far
                         as treating of Control-Z as being an
                         end-of-file marker.)

                 :mmap   A layer which implements "reading" of
                         files by using "mmap()" to make (whole)
                         file appear in the process's address
                         space, and then using that as PerlIO's

                 :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 nitroglycerin.

                 :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 any translation.  In particular
                         CRLF translation, and/or :utf8 intuited
                         from locale are disabled.

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

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                 :unix   Low level layer which calls "read",
                         "write" and "lseek" etc.

                 :utf8   A pseudolayer that turns on a flag on
                         the layer below to tell perl that output
                         should be in utf8 and that input should
                         be regarded as already in valid utf8
                         form. It does not check for validity and
                         as such should be handled with caution
                         for input. 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 unix-like numeric file descriptor
                         layer. Known to be buggy in this

                 On all platforms the default set of layers
                 should give acceptable results.

                 For Unix platforms that will equivalent of "unix
                 perlio" or "stdio".  Configure is setup to
                 prefer "stdio" implementation if system's
                 library provides for fast access to the buffer,
                 otherwise it uses the "unix perlio"

                 On Win32 the default in this release is "unix
                 crlf". Win32's "stdio" has a number of
                 bugs/mis-features for perl IO which are somewhat
                 C compiler vendor/version dependent. Using our
                 own "crlf" layer as the buffer avoids those
                 issues and makes things more uniform.  The
                 "crlf" layer provides CRLF to/from "\n"
                 conversion as well as buffering.

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

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

                 If set to the name of a file or device then

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                 certain operations of PerlIO sub-system will be
                 logged to that file (opened as append). Typical
                 uses are Unix:

                    PERLIO_DEBUG=/dev/tty perl script ...

                 and Win32 approximate 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 '' }

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

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

     PERL5SHELL (specific to the Win32 port)
                 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 " /c" on
                 Windows95.  The value is considered to be space-
                 separated.  Precede any character that needs to
                 be protected (like a space or backslash) with a

                 Note that Perl doesn't use COMSPEC for this
                 purpose because COMSPEC has a high degree of
                 variability among users, leading to portability
                 concerns.  Besides, perl can use a shell that
                 may not be fit for interactive use, and setting
                 COMSPEC to such a shell may interfere with the
                 proper functioning of other programs (which

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                 usually look in COMSPEC to find a shell fit for
                 interactive use).

                 Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not
                 taint checked when running external commands.
                 It is recommended that you explicitly set (or
                 delete) $ENV{PERL5SHELL} when running in taint
                 mode under Windows.

     PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
                 Set to 1 to allow the use of non-IFS compatible
                 LSP's.  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 all applications to use its LSP
                 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 some other games which allow
                 applications requiring IFS compatibility to

                 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 causes memory statistics to be dumped
                 after execution.  If set to an integer greater
                 than one, also causes memory statistics to be
                 dumped after compilation.

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

                 Set to one 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 it.

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

                 (Since Perl 5.8.1.)  Used to randomise perl's
                 internal hash function.  To emulate the
                 pre-5.8.1 behaviour, set to an integer (zero
                 means exactly the same order as 5.8.0).
                 "Pre-5.8.1" means, among other things, that hash
                 keys will always have the same ordering between
                 different runs of perl.

                 Most hashes return elements in the same order as
                 Perl 5.8.0 by default.  On a hash by hash basis,
                 if pathological data is detected during a hash
                 key insertion, then that hash will switch to an
                 alternative random hash seed.

                 The default behaviour is to randomise unless the
                 PERL_HASH_SEED is set.  If perl has been
                 compiled with "-DUSE_HASH_SEED_EXPLICIT", the
                 default behaviour is not to randomise unless the
                 PERL_HASH_SEED is set.

                 If PERL_HASH_SEED is unset or set to a non-
                 numeric string, perl uses the pseudorandom seed
                 supplied by the operating system and libraries.

                 Please note that the hash seed is sensitive
                 information. Hashes are randomized to protect
                 against local and remote attacks against Perl
                 code. By manually setting a seed this protection
                 may be partially or completely lost.

                 See "Algorithmic Complexity Attacks" in perlsec
                 and "PERL_HASH_SEED_DEBUG" for more information.

                 (Since Perl 5.8.1.)  Set to one to display (to
                 STDERR) the value of the hash seed at the
                 beginning of execution.  This, combined with
                 "PERL_HASH_SEED" is intended to aid in debugging
                 nondeterministic behavior caused by hash

                 Note that 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"

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                 in perlsec for more information.  Do not
                 disclose the hash seed to people who don't need
                 to know it.  See also hash_seed() of Hash::Util.

                 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
                 "<number>[m][s][t]", where "number" is the
                 filedescriptor number you want to write to (2 is
                 default), and the combination of letters
                 specifies that you want information about
                 (m)emory and/or (s)v, optionally with
                 (t)imestamps. For example "PERL_MEM_LOG=1mst"
                 will log all information to stdout. You can
                 write to other opened filedescriptors too, in a
                 variety of ways;

                   bash$ 3>foo3 PERL_MEM_LOG=3m perl ...

     PERL_ROOT (specific to the VMS port)
                 A translation concealed rooted logical name that
                 contains perl and the logical device for the
                 @INC path on VMS only.  Other logical names that
                 affect perl on VMS include PERLSHR,
                 but are optional and discussed further in
                 perlvms and in README.vms in the Perl source

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

                 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.

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

     Apart from these, Perl uses no other environment variables,
     except to make them available to the program being executed,
     and to child processes.  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)};

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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