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   1  =head1 NAME
   2  
   3  perlrun - how to execute the Perl interpreter
   4  
   5  =head1 SYNOPSIS
   6  
   7  B<perl>    S<[ B<-sTtuUWX> ]>
   8      S<[ B<-hv> ] [ B<-V>[:I<configvar>] ]>
   9      S<[ B<-cw> ] [ B<-d>[B<t>][:I<debugger>] ] [ B<-D>[I<number/list>] ]>
  10      S<[ B<-pna> ] [ B<-F>I<pattern> ] [ B<-l>[I<octal>] ] [ B<-0>[I<octal/hexadecimal>] ]>
  11      S<[ B<-I>I<dir> ] [ B<-m>[B<->]I<module> ] [ B<-M>[B<->]I<'module...'> ] [ B<-f> ]>
  12      S<[ B<-C [I<number/list>] >]>
  13      S<[ B<-P> ]>
  14      S<[ B<-S> ]>
  15      S<[ B<-x>[I<dir>] ]>
  16      S<[ B<-i>[I<extension>] ]>
  17      S<[ B<-eE> I<'command'> ] [ B<--> ] [ I<programfile> ] [ I<argument> ]...>
  18  
  19  =head1 DESCRIPTION
  20  
  21  The normal way to run a Perl program is by making it directly
  22  executable, or else by passing the name of the source file as an
  23  argument on the command line.  (An interactive Perl environment
  24  is also possible--see L<perldebug> for details on how to do that.)
  25  Upon startup, Perl looks for your program in one of the following
  26  places:
  27  
  28  =over 4
  29  
  30  =item 1.
  31  
  32  Specified line by line via B<-e> or B<-E> switches on the command line.
  33  
  34  =item 2.
  35  
  36  Contained in the file specified by the first filename on the command line.
  37  (Note that systems supporting the #! notation invoke interpreters this
  38  way. See L<Location of Perl>.)
  39  
  40  =item 3.
  41  
  42  Passed in implicitly via standard input.  This works only if there are
  43  no filename arguments--to pass arguments to a STDIN-read program you
  44  must explicitly specify a "-" for the program name.
  45  
  46  =back
  47  
  48  With methods 2 and 3, Perl starts parsing the input file from the
  49  beginning, unless you've specified a B<-x> switch, in which case it
  50  scans for the first line starting with #! and containing the word
  51  "perl", and starts there instead.  This is useful for running a program
  52  embedded in a larger message.  (In this case you would indicate the end
  53  of the program using the C<__END__> token.)
  54  
  55  The #! line is always examined for switches as the line is being
  56  parsed.  Thus, if you're on a machine that allows only one argument
  57  with the #! line, or worse, doesn't even recognize the #! line, you
  58  still can get consistent switch behavior regardless of how Perl was
  59  invoked, even if B<-x> was used to find the beginning of the program.
  60  
  61  Because historically some operating systems silently chopped off
  62  kernel interpretation of the #! line after 32 characters, some
  63  switches may be passed in on the command line, and some may not;
  64  you could even get a "-" without its letter, if you're not careful.
  65  You probably want to make sure that all your switches fall either
  66  before or after that 32-character boundary.  Most switches don't
  67  actually care if they're processed redundantly, but getting a "-"
  68  instead of a complete switch could cause Perl to try to execute
  69  standard input instead of your program.  And a partial B<-I> switch
  70  could also cause odd results.
  71  
  72  Some switches do care if they are processed twice, for instance
  73  combinations of B<-l> and B<-0>.  Either put all the switches after
  74  the 32-character boundary (if applicable), or replace the use of
  75  B<-0>I<digits> by C<BEGIN{ $/ = "\0digits"; }>.
  76  
  77  Parsing of the #! switches starts wherever "perl" is mentioned in the line.
  78  The sequences "-*" and "- " are specifically ignored so that you could,
  79  if you were so inclined, say
  80  
  81      #!/bin/sh -- # -*- perl -*- -p
  82      eval 'exec perl -wS $0 $1+"$@"}'
  83          if $running_under_some_shell;
  84  
  85  to let Perl see the B<-p> switch.
  86  
  87  A similar trick involves the B<env> program, if you have it.
  88  
  89      #!/usr/bin/env perl
  90  
  91  The examples above use a relative path to the perl interpreter,
  92  getting whatever version is first in the user's path.  If you want
  93  a specific version of Perl, say, perl5.005_57, you should place
  94  that directly in the #! line's path.
  95  
  96  If the #! line does not contain the word "perl", the program named after
  97  the #! is executed instead of the Perl interpreter.  This is slightly
  98  bizarre, but it helps people on machines that don't do #!, because they
  99  can tell a program that their SHELL is F</usr/bin/perl>, and Perl will then
 100  dispatch the program to the correct interpreter for them.
 101  
 102  After locating your program, Perl compiles the entire program to an
 103  internal form.  If there are any compilation errors, execution of the
 104  program is not attempted.  (This is unlike the typical shell script,
 105  which might run part-way through before finding a syntax error.)
 106  
 107  If the program is syntactically correct, it is executed.  If the program
 108  runs off the end without hitting an exit() or die() operator, an implicit
 109  C<exit(0)> is provided to indicate successful completion.
 110  
 111  =head2 #! and quoting on non-Unix systems
 112  X<hashbang> X<#!>
 113  
 114  Unix's #! technique can be simulated on other systems:
 115  
 116  =over 4
 117  
 118  =item OS/2
 119  
 120  Put
 121  
 122      extproc perl -S -your_switches
 123  
 124  as the first line in C<*.cmd> file (B<-S> due to a bug in cmd.exe's
 125  `extproc' handling).
 126  
 127  =item MS-DOS
 128  
 129  Create a batch file to run your program, and codify it in
 130  C<ALTERNATE_SHEBANG> (see the F<dosish.h> file in the source
 131  distribution for more information).
 132  
 133  =item Win95/NT
 134  
 135  The Win95/NT installation, when using the ActiveState installer for Perl,
 136  will modify the Registry to associate the F<.pl> extension with the perl
 137  interpreter.  If you install Perl by other means (including building from
 138  the sources), you may have to modify the Registry yourself.  Note that
 139  this means you can no longer tell the difference between an executable
 140  Perl program and a Perl library file.
 141  
 142  =item Macintosh
 143  
 144  Under "Classic" MacOS, a perl program will have the appropriate Creator and
 145  Type, so that double-clicking them will invoke the MacPerl application.
 146  Under Mac OS X, clickable apps can be made from any C<#!> script using Wil
 147  Sanchez' DropScript utility: http://www.wsanchez.net/software/ .
 148  
 149  =item VMS
 150  
 151  Put
 152  
 153      $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
 154      $ exit++ + ++$status != 0 and $exit = $status = undef;
 155  
 156  at the top of your program, where B<-mysw> are any command line switches you
 157  want to pass to Perl.  You can now invoke the program directly, by saying
 158  C<perl program>, or as a DCL procedure, by saying C<@program> (or implicitly
 159  via F<DCL$PATH> by just using the name of the program).
 160  
 161  This incantation is a bit much to remember, but Perl will display it for
 162  you if you say C<perl "-V:startperl">.
 163  
 164  =back
 165  
 166  Command-interpreters on non-Unix systems have rather different ideas
 167  on quoting than Unix shells.  You'll need to learn the special
 168  characters in your command-interpreter (C<*>, C<\> and C<"> are
 169  common) and how to protect whitespace and these characters to run
 170  one-liners (see B<-e> below).
 171  
 172  On some systems, you may have to change single-quotes to double ones,
 173  which you must I<not> do on Unix or Plan 9 systems.  You might also
 174  have to change a single % to a %%.
 175  
 176  For example:
 177  
 178      # Unix
 179      perl -e 'print "Hello world\n"'
 180  
 181      # MS-DOS, etc.
 182      perl -e "print \"Hello world\n\""
 183  
 184      # Macintosh
 185      print "Hello world\n"
 186       (then Run "Myscript" or Shift-Command-R)
 187  
 188      # VMS
 189      perl -e "print ""Hello world\n"""
 190  
 191  The problem is that none of this is reliable: it depends on the
 192  command and it is entirely possible neither works.  If B<4DOS> were
 193  the command shell, this would probably work better:
 194  
 195      perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
 196  
 197  B<CMD.EXE> in Windows NT slipped a lot of standard Unix functionality in
 198  when nobody was looking, but just try to find documentation for its
 199  quoting rules.
 200  
 201  Under the Macintosh, it depends which environment you are using.  The MacPerl
 202  shell, or MPW, is much like Unix shells in its support for several
 203  quoting variants, except that it makes free use of the Macintosh's non-ASCII
 204  characters as control characters.
 205  
 206  There is no general solution to all of this.  It's just a mess.
 207  
 208  =head2 Location of Perl
 209  X<perl, location of interpreter>
 210  
 211  It may seem obvious to say, but Perl is useful only when users can
 212  easily find it.  When possible, it's good for both F</usr/bin/perl>
 213  and F</usr/local/bin/perl> to be symlinks to the actual binary.  If
 214  that can't be done, system administrators are strongly encouraged
 215  to put (symlinks to) perl and its accompanying utilities into a
 216  directory typically found along a user's PATH, or in some other
 217  obvious and convenient place.
 218  
 219  In this documentation, C<#!/usr/bin/perl> on the first line of the program
 220  will stand in for whatever method works on your system.  You are
 221  advised to use a specific path if you care about a specific version.
 222  
 223      #!/usr/local/bin/perl5.00554
 224  
 225  or if you just want to be running at least version, place a statement
 226  like this at the top of your program:
 227  
 228      use 5.005_54;
 229  
 230  =head2 Command Switches
 231  X<perl, command switches> X<command switches>
 232  
 233  As with all standard commands, a single-character switch may be
 234  clustered with the following switch, if any.
 235  
 236      #!/usr/bin/perl -spi.orig    # same as -s -p -i.orig
 237  
 238  Switches include:
 239  
 240  =over 5
 241  
 242  =item B<-0>[I<octal/hexadecimal>]
 243  X<-0> X<$/>
 244  
 245  specifies the input record separator (C<$/>) as an octal or
 246  hexadecimal number.  If there are no digits, the null character is the
 247  separator.  Other switches may precede or follow the digits.  For
 248  example, if you have a version of B<find> which can print filenames
 249  terminated by the null character, you can say this:
 250  
 251      find . -name '*.orig' -print0 | perl -n0e unlink
 252  
 253  The special value 00 will cause Perl to slurp files in paragraph mode.
 254  The value 0777 will cause Perl to slurp files whole because there is no
 255  legal byte with that value.
 256  
 257  If you want to specify any Unicode character, use the hexadecimal
 258  format: C<-0xHHH...>, where the C<H> are valid hexadecimal digits.
 259  (This means that you cannot use the C<-x> with a directory name that
 260  consists of hexadecimal digits.)
 261  
 262  =item B<-a>
 263  X<-a> X<autosplit>
 264  
 265  turns on autosplit mode when used with a B<-n> or B<-p>.  An implicit
 266  split command to the @F array is done as the first thing inside the
 267  implicit while loop produced by the B<-n> or B<-p>.
 268  
 269      perl -ane 'print pop(@F), "\n";'
 270  
 271  is equivalent to
 272  
 273      while (<>) {
 274      @F = split(' ');
 275      print pop(@F), "\n";
 276      }
 277  
 278  An alternate delimiter may be specified using B<-F>.
 279  
 280  =item B<-C [I<number/list>]>
 281  X<-C>
 282  
 283  The C<-C> flag controls some of the Perl Unicode features.
 284  
 285  As of 5.8.1, the C<-C> can be followed either by a number or a list
 286  of option letters.  The letters, their numeric values, and effects
 287  are as follows; listing the letters is equal to summing the numbers.
 288  
 289      I     1   STDIN is assumed to be in UTF-8
 290      O     2   STDOUT will be in UTF-8
 291      E     4   STDERR will be in UTF-8
 292      S     7   I + O + E
 293      i     8   UTF-8 is the default PerlIO layer for input streams
 294      o    16   UTF-8 is the default PerlIO layer for output streams
 295      D    24   i + o
 296      A    32   the @ARGV elements are expected to be strings encoded
 297                in UTF-8
 298      L    64   normally the "IOEioA" are unconditional,
 299                the L makes them conditional on the locale environment
 300                variables (the LC_ALL, LC_TYPE, and LANG, in the order
 301                of decreasing precedence) -- if the variables indicate
 302                UTF-8, then the selected "IOEioA" are in effect
 303      a   256   Set ${^UTF8CACHE} to -1, to run the UTF-8 caching code in
 304                debugging mode.
 305  
 306  =for documenting_the_underdocumented
 307  perl.h gives W/128 as PERL_UNICODE_WIDESYSCALLS "/* for Sarathy */"
 308  
 309  =for todo
 310  perltodo mentions Unicode in %ENV and filenames. I guess that these will be
 311  options e and f (or F).
 312  
 313  For example, C<-COE> and C<-C6> will both turn on UTF-8-ness on both
 314  STDOUT and STDERR.  Repeating letters is just redundant, not cumulative
 315  nor toggling.
 316  
 317  The C<io> options mean that any subsequent open() (or similar I/O
 318  operations) will have the C<:utf8> PerlIO layer implicitly applied
 319  to them, in other words, UTF-8 is expected from any input stream,
 320  and UTF-8 is produced to any output stream.  This is just the default,
 321  with explicit layers in open() and with binmode() one can manipulate
 322  streams as usual.
 323  
 324  C<-C> on its own (not followed by any number or option list), or the
 325  empty string C<""> for the C<PERL_UNICODE> environment variable, has the
 326  same effect as C<-CSDL>.  In other words, the standard I/O handles and
 327  the default C<open()> layer are UTF-8-fied B<but> only if the locale
 328  environment variables indicate a UTF-8 locale.  This behaviour follows
 329  the I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
 330  
 331  You can use C<-C0> (or C<"0"> for C<PERL_UNICODE>) to explicitly
 332  disable all the above Unicode features.
 333  
 334  The read-only magic variable C<${^UNICODE}> reflects the numeric value
 335  of this setting.  This is variable is set during Perl startup and is
 336  thereafter read-only.  If you want runtime effects, use the three-arg
 337  open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
 338  and the C<open> pragma (see L<open>).
 339  
 340  (In Perls earlier than 5.8.1 the C<-C> switch was a Win32-only switch
 341  that enabled the use of Unicode-aware "wide system call" Win32 APIs.
 342  This feature was practically unused, however, and the command line
 343  switch was therefore "recycled".)
 344  
 345  =item B<-c>
 346  X<-c>
 347  
 348  causes Perl to check the syntax of the program and then exit without
 349  executing it.  Actually, it I<will> execute C<BEGIN>, C<UNITCHECK>,
 350  C<CHECK>, and C<use> blocks, because these are considered as occurring
 351  outside the execution of your program.  C<INIT> and C<END> blocks,
 352  however, will be skipped.
 353  
 354  =item B<-d>
 355  X<-d> X<-dt>
 356  
 357  =item B<-dt>
 358  
 359  runs the program under the Perl debugger.  See L<perldebug>.
 360  If B<t> is specified, it indicates to the debugger that threads
 361  will be used in the code being debugged.
 362  
 363  =item B<-d:>I<foo[=bar,baz]>
 364  X<-d> X<-dt>
 365  
 366  =item B<-dt:>I<foo[=bar,baz]>
 367  
 368  runs the program under the control of a debugging, profiling, or
 369  tracing module installed as Devel::foo. E.g., B<-d:DProf> executes
 370  the program using the Devel::DProf profiler.  As with the B<-M>
 371  flag, options may be passed to the Devel::foo package where they
 372  will be received and interpreted by the Devel::foo::import routine.
 373  The comma-separated list of options must follow a C<=> character.
 374  If B<t> is specified, it indicates to the debugger that threads
 375  will be used in the code being debugged.
 376  See L<perldebug>.
 377  
 378  =item B<-D>I<letters>
 379  X<-D> X<DEBUGGING> X<-DDEBUGGING>
 380  
 381  =item B<-D>I<number>
 382  
 383  sets debugging flags.  To watch how it executes your program, use
 384  B<-Dtls>.  (This works only if debugging is compiled into your
 385  Perl.)  Another nice value is B<-Dx>, which lists your compiled
 386  syntax tree.  And B<-Dr> displays compiled regular expressions;
 387  the format of the output is explained in L<perldebguts>.
 388  
 389  As an alternative, specify a number instead of list of letters (e.g.,
 390  B<-D14> is equivalent to B<-Dtls>):
 391  
 392          1  p  Tokenizing and parsing (with v, displays parse stack)
 393          2  s  Stack snapshots (with v, displays all stacks)
 394          4  l  Context (loop) stack processing
 395          8  t  Trace execution
 396         16  o  Method and overloading resolution
 397         32  c  String/numeric conversions
 398         64  P  Print profiling info, preprocessor command for -P, source file input state
 399        128  m  Memory allocation
 400        256  f  Format processing
 401        512  r  Regular expression parsing and execution
 402       1024  x  Syntax tree dump
 403       2048  u  Tainting checks
 404       4096  U  Unofficial, User hacking (reserved for private, unreleased use)
 405       8192  H  Hash dump -- usurps values()
 406      16384  X  Scratchpad allocation
 407      32768  D  Cleaning up
 408      65536  S  Thread synchronization
 409     131072  T  Tokenising
 410     262144  R  Include reference counts of dumped variables (eg when using -Ds)
 411     524288  J  Do not s,t,P-debug (Jump over) opcodes within package DB
 412    1048576  v  Verbose: use in conjunction with other flags
 413    2097152  C  Copy On Write
 414    4194304  A  Consistency checks on internal structures
 415    8388608  q  quiet - currently only suppresses the "EXECUTING" message
 416  
 417  All these flags require B<-DDEBUGGING> when you compile the Perl
 418  executable (but see L<Devel::Peek>, L<re> which may change this).
 419  See the F<INSTALL> file in the Perl source distribution
 420  for how to do this.  This flag is automatically set if you include B<-g>
 421  option when C<Configure> asks you about optimizer/debugger flags.
 422  
 423  If you're just trying to get a print out of each line of Perl code
 424  as it executes, the way that C<sh -x> provides for shell scripts,
 425  you can't use Perl's B<-D> switch.  Instead do this
 426  
 427    # If you have "env" utility
 428    env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
 429  
 430    # Bourne shell syntax
 431    $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
 432  
 433    # csh syntax
 434    % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
 435  
 436  See L<perldebug> for details and variations.
 437  
 438  =item B<-e> I<commandline>
 439  X<-e>
 440  
 441  may be used to enter one line of program.  If B<-e> is given, Perl
 442  will not look for a filename in the argument list.  Multiple B<-e>
 443  commands may be given to build up a multi-line script.  Make sure
 444  to use semicolons where you would in a normal program.
 445  
 446  =item B<-E> I<commandline>
 447  X<-E>
 448  
 449  behaves just like B<-e>, except that it implicitly enables all
 450  optional features (in the main compilation unit). See L<feature>.
 451  
 452  =item B<-f>
 453  X<-f>
 454  
 455  Disable executing F<$Config{sitelib}/sitecustomize.pl> at startup.
 456  
 457  Perl can be built so that it by default will try to execute
 458  F<$Config{sitelib}/sitecustomize.pl> at startup.  This is a hook that
 459  allows the sysadmin to customize how perl behaves.  It can for
 460  instance be used to add entries to the @INC array to make perl find
 461  modules in non-standard locations.
 462  
 463  =item B<-F>I<pattern>
 464  X<-F>
 465  
 466  specifies the pattern to split on if B<-a> is also in effect.  The
 467  pattern may be surrounded by C<//>, C<"">, or C<''>, otherwise it will be
 468  put in single quotes. You can't use literal whitespace in the pattern.
 469  
 470  =item B<-h>
 471  X<-h>
 472  
 473  prints a summary of the options.
 474  
 475  =item B<-i>[I<extension>]
 476  X<-i> X<in-place>
 477  
 478  specifies that files processed by the C<E<lt>E<gt>> construct are to be
 479  edited in-place.  It does this by renaming the input file, opening the
 480  output file by the original name, and selecting that output file as the
 481  default for print() statements.  The extension, if supplied, is used to
 482  modify the name of the old file to make a backup copy, following these
 483  rules:
 484  
 485  If no extension is supplied, no backup is made and the current file is
 486  overwritten.
 487  
 488  If the extension doesn't contain a C<*>, then it is appended to the
 489  end of the current filename as a suffix.  If the extension does
 490  contain one or more C<*> characters, then each C<*> is replaced
 491  with the current filename.  In Perl terms, you could think of this
 492  as:
 493  
 494      ($backup = $extension) =~ s/\*/$file_name/g;
 495  
 496  This allows you to add a prefix to the backup file, instead of (or in
 497  addition to) a suffix:
 498  
 499      $ perl -pi'orig_*' -e 's/bar/baz/' fileA    # backup to 'orig_fileA'
 500  
 501  Or even to place backup copies of the original files into another
 502  directory (provided the directory already exists):
 503  
 504      $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to 'old/fileA.orig'
 505  
 506  These sets of one-liners are equivalent:
 507  
 508      $ perl -pi -e 's/bar/baz/' fileA        # overwrite current file
 509      $ perl -pi'*' -e 's/bar/baz/' fileA        # overwrite current file
 510  
 511      $ perl -pi'.orig' -e 's/bar/baz/' fileA    # backup to 'fileA.orig'
 512      $ perl -pi'*.orig' -e 's/bar/baz/' fileA    # backup to 'fileA.orig'
 513  
 514  From the shell, saying
 515  
 516      $ perl -p -i.orig -e "s/foo/bar/; ... "
 517  
 518  is the same as using the program:
 519  
 520      #!/usr/bin/perl -pi.orig
 521      s/foo/bar/;
 522  
 523  which is equivalent to
 524  
 525      #!/usr/bin/perl
 526      $extension = '.orig';
 527      LINE: while (<>) {
 528      if ($ARGV ne $oldargv) {
 529          if ($extension !~ /\*/) {
 530          $backup = $ARGV . $extension;
 531          }
 532          else {
 533          ($backup = $extension) =~ s/\*/$ARGV/g;
 534          }
 535          rename($ARGV, $backup);
 536          open(ARGVOUT, ">$ARGV");
 537          select(ARGVOUT);
 538          $oldargv = $ARGV;
 539      }
 540      s/foo/bar/;
 541      }
 542      continue {
 543      print;    # this prints to original filename
 544      }
 545      select(STDOUT);
 546  
 547  except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
 548  know when the filename has changed.  It does, however, use ARGVOUT for
 549  the selected filehandle.  Note that STDOUT is restored as the default
 550  output filehandle after the loop.
 551  
 552  As shown above, Perl creates the backup file whether or not any output
 553  is actually changed.  So this is just a fancy way to copy files:
 554  
 555      $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
 556  or
 557      $ perl -p -i'.orig' -e 1 file1 file2 file3...
 558  
 559  You can use C<eof> without parentheses to locate the end of each input
 560  file, in case you want to append to each file, or reset line numbering
 561  (see example in L<perlfunc/eof>).
 562  
 563  If, for a given file, Perl is unable to create the backup file as
 564  specified in the extension then it will skip that file and continue on
 565  with the next one (if it exists).
 566  
 567  For a discussion of issues surrounding file permissions and B<-i>,
 568  see L<perlfaq5/Why does Perl let me delete read-only files?  Why does -i clobber protected files?  Isn't this a bug in Perl?>.
 569  
 570  You cannot use B<-i> to create directories or to strip extensions from
 571  files.
 572  
 573  Perl does not expand C<~> in filenames, which is good, since some
 574  folks use it for their backup files:
 575  
 576      $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
 577  
 578  Note that because B<-i> renames or deletes the original file before
 579  creating a new file of the same name, UNIX-style soft and hard links will
 580  not be preserved.
 581  
 582  Finally, the B<-i> switch does not impede execution when no
 583  files are given on the command line.  In this case, no backup is made
 584  (the original file cannot, of course, be determined) and processing
 585  proceeds from STDIN to STDOUT as might be expected.
 586  
 587  =item B<-I>I<directory>
 588  X<-I> X<@INC>
 589  
 590  Directories specified by B<-I> are prepended to the search path for
 591  modules (C<@INC>), and also tells the C preprocessor where to search for
 592  include files.  The C preprocessor is invoked with B<-P>; by default it
 593  searches /usr/include and /usr/lib/perl.
 594  
 595  =item B<-l>[I<octnum>]
 596  X<-l> X<$/> X<$\>
 597  
 598  enables automatic line-ending processing.  It has two separate
 599  effects.  First, it automatically chomps C<$/> (the input record
 600  separator) when used with B<-n> or B<-p>.  Second, it assigns C<$\>
 601  (the output record separator) to have the value of I<octnum> so
 602  that any print statements will have that separator added back on.
 603  If I<octnum> is omitted, sets C<$\> to the current value of
 604  C<$/>.  For instance, to trim lines to 80 columns:
 605  
 606      perl -lpe 'substr($_, 80) = ""'
 607  
 608  Note that the assignment C<$\ = $/> is done when the switch is processed,
 609  so the input record separator can be different than the output record
 610  separator if the B<-l> switch is followed by a B<-0> switch:
 611  
 612      gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
 613  
 614  This sets C<$\> to newline and then sets C<$/> to the null character.
 615  
 616  =item B<-m>[B<->]I<module>
 617  X<-m> X<-M>
 618  
 619  =item B<-M>[B<->]I<module>
 620  
 621  =item B<-M>[B<->]I<'module ...'>
 622  
 623  =item B<-[mM]>[B<->]I<module=arg[,arg]...>
 624  
 625  B<-m>I<module> executes C<use> I<module> C<();> before executing your
 626  program.
 627  
 628  B<-M>I<module> executes C<use> I<module> C<;> before executing your
 629  program.  You can use quotes to add extra code after the module name,
 630  e.g., C<'-Mmodule qw(foo bar)'>.
 631  
 632  If the first character after the B<-M> or B<-m> is a dash (C<->)
 633  then the 'use' is replaced with 'no'.
 634  
 635  A little builtin syntactic sugar means you can also say
 636  B<-mmodule=foo,bar> or B<-Mmodule=foo,bar> as a shortcut for
 637  C<'-Mmodule qw(foo bar)'>.  This avoids the need to use quotes when
 638  importing symbols.  The actual code generated by B<-Mmodule=foo,bar> is
 639  C<use module split(/,/,q{foo,bar})>.  Note that the C<=> form
 640  removes the distinction between B<-m> and B<-M>.
 641  
 642  A consequence of this is that B<-MFoo=number> never does a version check
 643  (unless C<Foo::import()> itself is set up to do a version check, which
 644  could happen for example if Foo inherits from Exporter.)
 645  
 646  =item B<-n>
 647  X<-n>
 648  
 649  causes Perl to assume the following loop around your program, which
 650  makes it iterate over filename arguments somewhat like B<sed -n> or
 651  B<awk>:
 652  
 653    LINE:
 654      while (<>) {
 655      ...        # your program goes here
 656      }
 657  
 658  Note that the lines are not printed by default.  See B<-p> to have
 659  lines printed.  If a file named by an argument cannot be opened for
 660  some reason, Perl warns you about it and moves on to the next file.
 661  
 662  Here is an efficient way to delete all files that haven't been modified for
 663  at least a week:
 664  
 665      find . -mtime +7 -print | perl -nle unlink
 666  
 667  This is faster than using the B<-exec> switch of B<find> because you don't
 668  have to start a process on every filename found.  It does suffer from
 669  the bug of mishandling newlines in pathnames, which you can fix if
 670  you follow the example under B<-0>.
 671  
 672  C<BEGIN> and C<END> blocks may be used to capture control before or after
 673  the implicit program loop, just as in B<awk>.
 674  
 675  =item B<-p>
 676  X<-p>
 677  
 678  causes Perl to assume the following loop around your program, which
 679  makes it iterate over filename arguments somewhat like B<sed>:
 680  
 681  
 682    LINE:
 683      while (<>) {
 684      ...        # your program goes here
 685      } continue {
 686      print or die "-p destination: $!\n";
 687      }
 688  
 689  If a file named by an argument cannot be opened for some reason, Perl
 690  warns you about it, and moves on to the next file.  Note that the
 691  lines are printed automatically.  An error occurring during printing is
 692  treated as fatal.  To suppress printing use the B<-n> switch.  A B<-p>
 693  overrides a B<-n> switch.
 694  
 695  C<BEGIN> and C<END> blocks may be used to capture control before or after
 696  the implicit loop, just as in B<awk>.
 697  
 698  =item B<-P>
 699  X<-P>
 700  
 701  B<NOTE: Use of -P is strongly discouraged because of its inherent
 702  problems, including poor portability. It is deprecated and will be
 703  removed in a future version of Perl.>
 704  
 705  This option causes your program to be run through the C preprocessor before
 706  compilation by Perl.  Because both comments and B<cpp> directives begin
 707  with the # character, you should avoid starting comments with any words
 708  recognized by the C preprocessor such as C<"if">, C<"else">, or C<"define">.
 709  
 710  If you're considering using C<-P>, you might also want to look at the
 711  Filter::cpp module from CPAN.
 712  
 713  The problems of -P include, but are not limited to:
 714  
 715  =over 10
 716  
 717  =item *
 718  
 719  The C<#!> line is stripped, so any switches there don't apply.
 720  
 721  =item *
 722  
 723  A C<-P> on a C<#!> line doesn't work.
 724  
 725  =item *
 726  
 727  B<All> lines that begin with (whitespace and) a C<#> but
 728  do not look like cpp commands, are stripped, including anything
 729  inside Perl strings, regular expressions, and here-docs .
 730  
 731  =item *
 732  
 733  In some platforms the C preprocessor knows too much: it knows about
 734  the C++ -style until-end-of-line comments starting with C<"//">.
 735  This will cause problems with common Perl constructs like
 736  
 737      s/foo//;
 738  
 739  because after -P this will became illegal code
 740  
 741      s/foo
 742  
 743  The workaround is to use some other quoting separator than C<"/">,
 744  like for example C<"!">:
 745  
 746      s!foo!!;
 747  
 748  
 749  
 750  =item *
 751  
 752  It requires not only a working C preprocessor but also a working
 753  F<sed>.  If not on UNIX, you are probably out of luck on this.
 754  
 755  =item *
 756  
 757  Script line numbers are not preserved.
 758  
 759  =item *
 760  
 761  The C<-x> does not work with C<-P>.
 762  
 763  =back
 764  
 765  =item B<-s>
 766  X<-s>
 767  
 768  enables rudimentary switch parsing for switches on the command
 769  line after the program name but before any filename arguments (or before
 770  an argument of B<-->).  Any switch found there is removed from @ARGV and sets the
 771  corresponding variable in the Perl program.  The following program
 772  prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
 773  if it is invoked with B<-xyz=abc>.
 774  
 775      #!/usr/bin/perl -s
 776      if ($xyz) { print "$xyz\n" }
 777  
 778  Do note that a switch like B<--help> creates the variable ${-help}, which is not compliant
 779  with C<strict refs>.  Also, when using this option on a script with
 780  warnings enabled you may get a lot of spurious "used only once" warnings.
 781  
 782  =item B<-S>
 783  X<-S>
 784  
 785  makes Perl use the PATH environment variable to search for the
 786  program (unless the name of the program contains directory separators).
 787  
 788  On some platforms, this also makes Perl append suffixes to the
 789  filename while searching for it.  For example, on Win32 platforms,
 790  the ".bat" and ".cmd" suffixes are appended if a lookup for the
 791  original name fails, and if the name does not already end in one
 792  of those suffixes.  If your Perl was compiled with DEBUGGING turned
 793  on, using the -Dp switch to Perl shows how the search progresses.
 794  
 795  Typically this is used to emulate #! startup on platforms that don't
 796  support #!.  Its also convenient when debugging a script that uses #!,
 797  and is thus normally found by the shell's $PATH search mechanism.
 798  
 799  This example works on many platforms that have a shell compatible with
 800  Bourne shell:
 801  
 802      #!/usr/bin/perl
 803      eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
 804          if $running_under_some_shell;
 805  
 806  The system ignores the first line and feeds the program to F</bin/sh>,
 807  which proceeds to try to execute the Perl program as a shell script.
 808  The shell executes the second line as a normal shell command, and thus
 809  starts up the Perl interpreter.  On some systems $0 doesn't always
 810  contain the full pathname, so the B<-S> tells Perl to search for the
 811  program if necessary.  After Perl locates the program, it parses the
 812  lines and ignores them because the variable $running_under_some_shell
 813  is never true.  If the program will be interpreted by csh, you will need
 814  to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
 815  embedded spaces (and such) in the argument list.  To start up sh rather
 816  than csh, some systems may have to replace the #! line with a line
 817  containing just a colon, which will be politely ignored by Perl.  Other
 818  systems can't control that, and need a totally devious construct that
 819  will work under any of B<csh>, B<sh>, or Perl, such as the following:
 820  
 821      eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
 822      & eval 'exec /usr/bin/perl -wS $0 $argv:q'
 823          if $running_under_some_shell;
 824  
 825  If the filename supplied contains directory separators (i.e., is an
 826  absolute or relative pathname), and if that file is not found,
 827  platforms that append file extensions will do so and try to look
 828  for the file with those extensions added, one by one.
 829  
 830  On DOS-like platforms, if the program does not contain directory
 831  separators, it will first be searched for in the current directory
 832  before being searched for on the PATH.  On Unix platforms, the
 833  program will be searched for strictly on the PATH.
 834  
 835  =item B<-t>
 836  X<-t>
 837  
 838  Like B<-T>, but taint checks will issue warnings rather than fatal
 839  errors.  These warnings can be controlled normally with C<no warnings
 840  qw(taint)>.
 841  
 842  B<NOTE: this is not a substitute for -T.> This is meant only to be
 843  used as a temporary development aid while securing legacy code:
 844  for real production code and for new secure code written from scratch
 845  always use the real B<-T>.
 846  
 847  =item B<-T>
 848  X<-T>
 849  
 850  forces "taint" checks to be turned on so you can test them.  Ordinarily
 851  these checks are done only when running setuid or setgid.  It's a
 852  good idea to turn them on explicitly for programs that run on behalf
 853  of someone else whom you might not necessarily trust, such as CGI
 854  programs or any internet servers you might write in Perl.  See
 855  L<perlsec> for details.  For security reasons, this option must be
 856  seen by Perl quite early; usually this means it must appear early
 857  on the command line or in the #! line for systems which support
 858  that construct.
 859  
 860  =item B<-u>
 861  X<-u>
 862  
 863  This obsolete switch causes Perl to dump core after compiling your
 864  program.  You can then in theory take this core dump and turn it
 865  into an executable file by using the B<undump> program (not supplied).
 866  This speeds startup at the expense of some disk space (which you
 867  can minimize by stripping the executable).  (Still, a "hello world"
 868  executable comes out to about 200K on my machine.)  If you want to
 869  execute a portion of your program before dumping, use the dump()
 870  operator instead.  Note: availability of B<undump> is platform
 871  specific and may not be available for a specific port of Perl.
 872  
 873  =item B<-U>
 874  X<-U>
 875  
 876  allows Perl to do unsafe operations.  Currently the only "unsafe"
 877  operations are attempting to unlink directories while running as 
 878  superuser, and running setuid programs with fatal taint checks turned
 879  into warnings.  Note that the B<-w> switch (or the C<$^W> variable) 
 880  must be used along with this option to actually I<generate> the
 881  taint-check warnings.  
 882  
 883  =item B<-v>
 884  X<-v>
 885  
 886  prints the version and patchlevel of your perl executable.
 887  
 888  =item B<-V>
 889  X<-V>
 890  
 891  prints summary of the major perl configuration values and the current
 892  values of @INC.
 893  
 894  =item B<-V:>I<configvar>
 895  
 896  Prints to STDOUT the value of the named configuration variable(s),
 897  with multiples when your configvar argument looks like a regex (has
 898  non-letters).  For example:
 899  
 900      $ perl -V:libc
 901      libc='/lib/libc-2.2.4.so';
 902      $ perl -V:lib.
 903      libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
 904      libc='/lib/libc-2.2.4.so';
 905      $ perl -V:lib.*
 906      libpth='/usr/local/lib /lib /usr/lib';
 907      libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
 908      lib_ext='.a';
 909      libc='/lib/libc-2.2.4.so';
 910      libperl='libperl.a';
 911      ....
 912  
 913  Additionally, extra colons can be used to control formatting.  A
 914  trailing colon suppresses the linefeed and terminator ';', allowing
 915  you to embed queries into shell commands.  (mnemonic: PATH separator
 916  ':'.)
 917  
 918      $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
 919      compression-vars:  zcat='' zip='zip'  are here !
 920  
 921  A leading colon removes the 'name=' part of the response, this allows
 922  you to map to the name you need.  (mnemonic: empty label)
 923  
 924      $ echo "goodvfork="`./perl -Ilib -V::usevfork`
 925      goodvfork=false;
 926  
 927  Leading and trailing colons can be used together if you need
 928  positional parameter values without the names.  Note that in the case
 929  below, the PERL_API params are returned in alphabetical order.
 930  
 931      $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
 932      building_on 'linux' '5' '1' '9' now
 933  
 934  =item B<-w>
 935  X<-w>
 936  
 937  prints warnings about dubious constructs, such as variable names
 938  that are mentioned only once and scalar variables that are used
 939  before being set, redefined subroutines, references to undefined
 940  filehandles or filehandles opened read-only that you are attempting
 941  to write on, values used as a number that don't look like numbers,
 942  using an array as though it were a scalar, if your subroutines
 943  recurse more than 100 deep, and innumerable other things.
 944  
 945  This switch really just enables the internal C<$^W> variable.  You
 946  can disable or promote into fatal errors specific warnings using
 947  C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
 948  See also L<perldiag> and L<perltrap>.  A new, fine-grained warning
 949  facility is also available if you want to manipulate entire classes
 950  of warnings; see L<warnings> or L<perllexwarn>.
 951  
 952  =item B<-W>
 953  X<-W>
 954  
 955  Enables all warnings regardless of C<no warnings> or C<$^W>.
 956  See L<perllexwarn>.
 957  
 958  =item B<-X>
 959  X<-X>
 960  
 961  Disables all warnings regardless of C<use warnings> or C<$^W>.
 962  See L<perllexwarn>.
 963  
 964  =item B<-x>
 965  X<-x>
 966  
 967  =item B<-x>I<directory>
 968  
 969  tells Perl that the program is embedded in a larger chunk of unrelated
 970  ASCII text, such as in a mail message.  Leading garbage will be
 971  discarded until the first line that starts with #! and contains the
 972  string "perl".  Any meaningful switches on that line will be applied.
 973  If a directory name is specified, Perl will switch to that directory
 974  before running the program.  The B<-x> switch controls only the
 975  disposal of leading garbage.  The program must be terminated with
 976  C<__END__> if there is trailing garbage to be ignored (the program
 977  can process any or all of the trailing garbage via the DATA filehandle
 978  if desired).
 979  
 980  The directory, if specified, must appear immediately following the B<-x>
 981  with no intervening whitespace.
 982  
 983  =back
 984  
 985  =head1 ENVIRONMENT
 986  X<perl, environment variables>
 987  
 988  =over 12
 989  
 990  =item HOME
 991  X<HOME>
 992  
 993  Used if chdir has no argument.
 994  
 995  =item LOGDIR
 996  X<LOGDIR>
 997  
 998  Used if chdir has no argument and HOME is not set.
 999  
1000  =item PATH
1001  X<PATH>
1002  
1003  Used in executing subprocesses, and in finding the program if B<-S> is
1004  used.
1005  
1006  =item PERL5LIB
1007  X<PERL5LIB>
1008  
1009  A list of directories in which to look for Perl library
1010  files before looking in the standard library and the current
1011  directory.  Any architecture-specific directories under the specified
1012  locations are automatically included if they exist (this lookup
1013  being done at interpreter startup time.)
1014  
1015  If PERL5LIB is not defined, PERLLIB is used.  Directories are separated
1016  (like in PATH) by a colon on unixish platforms and by a semicolon on
1017  Windows (the proper path separator being given by the command C<perl
1018  -V:path_sep>).
1019  
1020  When running taint checks (either because the program was running setuid
1021  or setgid, or the B<-T> or B<-t> switch was specified), neither variable
1022  is used. The program should instead say:
1023  
1024      use lib "/my/directory";
1025  
1026  =item PERL5OPT
1027  X<PERL5OPT>
1028  
1029  Command-line options (switches).  Switches in this variable are taken
1030  as if they were on every Perl command line.  Only the B<-[CDIMUdmtw]>
1031  switches are allowed.  When running taint checks (because the program
1032  was running setuid or setgid, or the B<-T> switch was used), this
1033  variable is ignored.  If PERL5OPT begins with B<-T>, tainting will be
1034  enabled, and any subsequent options ignored.
1035  
1036  =item PERLIO
1037  X<PERLIO>
1038  
1039  A space (or colon) separated list of PerlIO layers. If perl is built
1040  to use PerlIO system for IO (the default) these layers effect perl's IO.
1041  
1042  It is conventional to start layer names with a colon e.g. C<:perlio> to
1043  emphasise their similarity to variable "attributes". But the code that parses
1044  layer specification strings (which is also used to decode the PERLIO
1045  environment variable) treats the colon as a separator.
1046  
1047  An unset or empty PERLIO is equivalent to the default set of layers for
1048  your platform, for example C<:unix:perlio> on UNIX-like systems
1049  and C<:unix:crlf> on Windows and other DOS-like systems.
1050  
1051  The list becomes the default for I<all> perl's IO. Consequently only built-in
1052  layers can appear in this list, as external layers (such as :encoding()) need
1053  IO in  order to load them!. See L<"open pragma"|open> for how to add external
1054  encodings as defaults.
1055  
1056  The layers that it makes sense to include in the PERLIO environment
1057  variable are briefly summarised below. For more details see L<PerlIO>.
1058  
1059  =over 8
1060  
1061  =item :bytes
1062  X<:bytes>
1063  
1064  A pseudolayer that turns I<off> the C<:utf8> flag for the layer below.
1065  Unlikely to be useful on its own in the global PERLIO environment variable.
1066  You perhaps were thinking of C<:crlf:bytes> or C<:perlio:bytes>.
1067  
1068  =item :crlf
1069  X<:crlf>
1070  
1071  A layer which does CRLF to "\n" translation distinguishing "text" and
1072  "binary" files in the manner of MS-DOS and similar operating systems.
1073  (It currently does I<not> mimic MS-DOS as far as treating of Control-Z
1074  as being an end-of-file marker.)
1075  
1076  =item :mmap
1077  X<:mmap>
1078  
1079  A layer which implements "reading" of files by using C<mmap()> to
1080  make (whole) file appear in the process's address space, and then
1081  using that as PerlIO's "buffer".
1082  
1083  =item :perlio
1084  X<:perlio>
1085  
1086  This is a re-implementation of "stdio-like" buffering written as a
1087  PerlIO "layer".  As such it will call whatever layer is below it for
1088  its operations (typically C<:unix>).
1089  
1090  =item :pop
1091  X<:pop>
1092  
1093  An experimental pseudolayer that removes the topmost layer.
1094  Use with the same care as is reserved for nitroglycerin.
1095  
1096  =item :raw
1097  X<:raw>
1098  
1099  A pseudolayer that manipulates other layers.  Applying the C<:raw>
1100  layer is equivalent to calling C<binmode($fh)>.  It makes the stream
1101  pass each byte as-is without any translation.  In particular CRLF
1102  translation, and/or :utf8 intuited from locale are disabled.
1103  
1104  Unlike in the earlier versions of Perl C<:raw> is I<not>
1105  just the inverse of C<:crlf> - other layers which would affect the
1106  binary nature of the stream are also removed or disabled.
1107  
1108  =item :stdio
1109  X<:stdio>
1110  
1111  This layer provides PerlIO interface by wrapping system's ANSI C "stdio"
1112  library calls. The layer provides both buffering and IO.
1113  Note that C<:stdio> layer does I<not> do CRLF translation even if that
1114  is platforms normal behaviour. You will need a C<:crlf> layer above it
1115  to do that.
1116  
1117  =item :unix
1118  X<:unix>
1119  
1120  Low level layer which calls C<read>, C<write> and C<lseek> etc.
1121  
1122  =item :utf8
1123  X<:utf8>
1124  
1125  A pseudolayer that turns on a flag on the layer below to tell perl
1126  that output should be in utf8 and that input should be regarded as
1127  already in valid utf8 form. It does not check for validity and as such
1128  should be handled with caution for input. Generally C<:encoding(utf8)> is
1129  the best option when reading UTF-8 encoded data.
1130  
1131  =item :win32
1132  X<:win32>
1133  
1134  On Win32 platforms this I<experimental> layer uses native "handle" IO
1135  rather than unix-like numeric file descriptor layer. Known to be
1136  buggy in this release.
1137  
1138  =back
1139  
1140  On all platforms the default set of layers should give acceptable results.
1141  
1142  For UNIX platforms that will equivalent of "unix perlio" or "stdio".
1143  Configure is setup to prefer "stdio" implementation if system's library
1144  provides for fast access to the buffer, otherwise it uses the "unix perlio"
1145  implementation.
1146  
1147  On Win32 the default in this release is "unix crlf". Win32's "stdio"
1148  has a number of bugs/mis-features for perl IO which are somewhat
1149  C compiler vendor/version dependent. Using our own C<crlf> layer as
1150  the buffer avoids those issues and makes things more uniform.
1151  The C<crlf> layer provides CRLF to/from "\n" conversion as well as
1152  buffering.
1153  
1154  This release uses C<unix> as the bottom layer on Win32 and so still uses C
1155  compiler's numeric file descriptor routines. There is an experimental native
1156  C<win32> layer which is expected to be enhanced and should eventually be
1157  the default under Win32.
1158  
1159  =item PERLIO_DEBUG
1160  X<PERLIO_DEBUG>
1161  
1162  If set to the name of a file or device then certain operations of PerlIO
1163  sub-system will be logged to that file (opened as append). Typical uses
1164  are UNIX:
1165  
1166     PERLIO_DEBUG=/dev/tty perl script ...
1167  
1168  and Win32 approximate equivalent:
1169  
1170     set PERLIO_DEBUG=CON
1171     perl script ...
1172  
1173  This functionality is disabled for setuid scripts and for scripts run
1174  with B<-T>.
1175  
1176  =item PERLLIB
1177  X<PERLLIB>
1178  
1179  A list of directories in which to look for Perl library
1180  files before looking in the standard library and the current directory.
1181  If PERL5LIB is defined, PERLLIB is not used.
1182  
1183  =item PERL5DB
1184  X<PERL5DB>
1185  
1186  The command used to load the debugger code.  The default is:
1187  
1188      BEGIN { require 'perl5db.pl' }
1189  
1190  =item PERL5DB_THREADED
1191  X<PERL5DB_THREADED>
1192  
1193  If set to a true value, indicates to the debugger that the code being
1194  debugged uses threads.
1195  
1196  =item PERL5SHELL (specific to the Win32 port)
1197  X<PERL5SHELL>
1198  
1199  May be set to an alternative shell that perl must use internally for
1200  executing "backtick" commands or system().  Default is C<cmd.exe /x/d/c>
1201  on WindowsNT and C<command.com /c> on Windows95.  The value is considered
1202  to be space-separated.  Precede any character that needs to be protected
1203  (like a space or backslash) with a backslash.
1204  
1205  Note that Perl doesn't use COMSPEC for this purpose because
1206  COMSPEC has a high degree of variability among users, leading to
1207  portability concerns.  Besides, perl can use a shell that may not be
1208  fit for interactive use, and setting COMSPEC to such a shell may
1209  interfere with the proper functioning of other programs (which usually
1210  look in COMSPEC to find a shell fit for interactive use).
1211  
1212  =item PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
1213  X<PERL_ALLOW_NON_IFS_LSP>
1214  
1215  Set to 1 to allow the use of non-IFS compatible LSP's.
1216  Perl normally searches for an IFS-compatible LSP because this is required
1217  for its emulation of Windows sockets as real filehandles.  However, this may
1218  cause problems if you have a firewall such as McAfee Guardian which requires
1219  all applications to use its LSP which is not IFS-compatible, because clearly
1220  Perl will normally avoid using such an LSP.
1221  Setting this environment variable to 1 means that Perl will simply use the
1222  first suitable LSP enumerated in the catalog, which keeps McAfee Guardian
1223  happy (and in that particular case Perl still works too because McAfee
1224  Guardian's LSP actually plays some other games which allow applications
1225  requiring IFS compatibility to work).
1226  
1227  =item PERL_DEBUG_MSTATS
1228  X<PERL_DEBUG_MSTATS>
1229  
1230  Relevant only if perl is compiled with the malloc included with the perl
1231  distribution (that is, if C<perl -V:d_mymalloc> is 'define').
1232  If set, this causes memory statistics to be dumped after execution.  If set
1233  to an integer greater than one, also causes memory statistics to be dumped
1234  after compilation.
1235  
1236  =item PERL_DESTRUCT_LEVEL
1237  X<PERL_DESTRUCT_LEVEL>
1238  
1239  Relevant only if your perl executable was built with B<-DDEBUGGING>,
1240  this controls the behavior of global destruction of objects and other
1241  references.  See L<perlhack/PERL_DESTRUCT_LEVEL> for more information.
1242  
1243  =item PERL_DL_NONLAZY
1244  X<PERL_DL_NONLAZY>
1245  
1246  Set to one to have perl resolve B<all> undefined symbols when it loads
1247  a dynamic library.  The default behaviour is to resolve symbols when
1248  they are used.  Setting this variable is useful during testing of
1249  extensions as it ensures that you get an error on misspelled function
1250  names even if the test suite doesn't call it.
1251  
1252  =item PERL_ENCODING
1253  X<PERL_ENCODING>
1254  
1255  If using the C<encoding> pragma without an explicit encoding name, the
1256  PERL_ENCODING environment variable is consulted for an encoding name.
1257  
1258  =item PERL_HASH_SEED
1259  X<PERL_HASH_SEED>
1260  
1261  (Since Perl 5.8.1.)  Used to randomise perl's internal hash function.
1262  To emulate the pre-5.8.1 behaviour, set to an integer (zero means
1263  exactly the same order as 5.8.0).  "Pre-5.8.1" means, among other
1264  things, that hash keys will always have the same ordering between
1265  different runs of perl.
1266  
1267  Most hashes return elements in the same order as Perl 5.8.0 by default.
1268  On a hash by hash basis, if pathological data is detected during a hash
1269  key insertion, then that hash will switch to an alternative random hash
1270  seed.
1271  
1272  The default behaviour is to randomise unless the PERL_HASH_SEED is set.
1273  If perl has been compiled with C<-DUSE_HASH_SEED_EXPLICIT>, the default
1274  behaviour is B<not> to randomise unless the PERL_HASH_SEED is set.
1275  
1276  If PERL_HASH_SEED is unset or set to a non-numeric string, perl uses
1277  the pseudorandom seed supplied by the operating system and libraries.
1278  
1279  B<Please note that the hash seed is sensitive information>. Hashes are
1280  randomized to protect against local and remote attacks against Perl
1281  code. By manually setting a seed this protection may be partially or
1282  completely lost.
1283  
1284  See L<perlsec/"Algorithmic Complexity Attacks"> and
1285  L</PERL_HASH_SEED_DEBUG> for more information.
1286  
1287  =item PERL_HASH_SEED_DEBUG
1288  X<PERL_HASH_SEED_DEBUG>
1289  
1290  (Since Perl 5.8.1.)  Set to one to display (to STDERR) the value of
1291  the hash seed at the beginning of execution.  This, combined with
1292  L</PERL_HASH_SEED> is intended to aid in debugging nondeterministic
1293  behavior caused by hash randomization.
1294  
1295  B<Note that the hash seed is sensitive information>: by knowing it one
1296  can craft a denial-of-service attack against Perl code, even remotely,
1297  see L<perlsec/"Algorithmic Complexity Attacks"> for more information.
1298  B<Do not disclose the hash seed> to people who don't need to know it.
1299  See also hash_seed() of L<Hash::Util>.
1300  
1301  =item PERL_ROOT (specific to the VMS port)
1302  X<PERL_ROOT>
1303  
1304  A translation concealed rooted logical name that contains perl and the
1305  logical device for the @INC path on VMS only.  Other logical names that
1306  affect perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1307  SYS$TIMEZONE_DIFFERENTIAL but are optional and discussed further in
1308  L<perlvms> and in F<README.vms> in the Perl source distribution.
1309  
1310  =item PERL_SIGNALS
1311  X<PERL_SIGNALS>
1312  
1313  In Perls 5.8.1 and later.  If set to C<unsafe> the pre-Perl-5.8.0
1314  signals behaviour (immediate but unsafe) is restored.  If set to
1315  C<safe> the safe (or deferred) signals are used.
1316  See L<perlipc/"Deferred Signals (Safe Signals)">.
1317  
1318  =item PERL_UNICODE
1319  X<PERL_UNICODE>
1320  
1321  Equivalent to the B<-C> command-line switch.  Note that this is not
1322  a boolean variable-- setting this to C<"1"> is not the right way to
1323  "enable Unicode" (whatever that would mean).  You can use C<"0"> to
1324  "disable Unicode", though (or alternatively unset PERL_UNICODE in
1325  your shell before starting Perl).  See the description of the C<-C>
1326  switch for more information.
1327  
1328  =item SYS$LOGIN (specific to the VMS port)
1329  X<SYS$LOGIN>
1330  
1331  Used if chdir has no argument and HOME and LOGDIR are not set.
1332  
1333  =back
1334  
1335  Perl also has environment variables that control how Perl handles data
1336  specific to particular natural languages.  See L<perllocale>.
1337  
1338  Apart from these, Perl uses no other environment variables, except
1339  to make them available to the program being executed, and to child
1340  processes.  However, programs running setuid would do well to execute
1341  the following lines before doing anything else, just to keep people
1342  honest:
1343  
1344      $ENV{PATH}  = '/bin:/usr/bin';    # or whatever you need
1345      $ENV{SHELL} = '/bin/sh' if exists $ENV{SHELL};
1346      delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};