3 perlrun - how to execute the Perl interpreter
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>] >]>
15 S<[ B<-i>[I<extension>] ]>
16 S<[ [B<-e>|B<-E>] I<'command'> ] [ B<--> ] [ I<programfile> ] [ I<argument> ]...>
20 The normal way to run a Perl program is by making it directly
21 executable, or else by passing the name of the source file as an
22 argument on the command line. (An interactive Perl environment
23 is also possible--see L<perldebug> for details on how to do that.)
24 Upon startup, Perl looks for your program in one of the following
31 Specified line by line via B<-e> or B<-E> switches on the command line.
35 Contained in the file specified by the first filename on the command line.
36 (Note that systems supporting the C<#!> notation invoke interpreters this
37 way. See L</Location of Perl>.)
41 Passed in implicitly via standard input. This works only if there are
42 no filename arguments--to pass arguments to a STDIN-read program you
43 must explicitly specify a "-" for the program name.
47 With methods 2 and 3, Perl starts parsing the input file from the
48 beginning, unless you've specified a B<-x> switch, in which case it
49 scans for the first line starting with C<#!> and containing the word
50 "perl", and starts there instead. This is useful for running a program
51 embedded in a larger message. (In this case you would indicate the end
52 of the program using the C<__END__> token.)
54 The C<#!> line is always examined for switches as the line is being
55 parsed. Thus, if you're on a machine that allows only one argument
56 with the C<#!> line, or worse, doesn't even recognize the C<#!> line, you
57 still can get consistent switch behaviour regardless of how Perl was
58 invoked, even if B<-x> was used to find the beginning of the program.
60 Because historically some operating systems silently chopped off
61 kernel interpretation of the C<#!> line after 32 characters, some
62 switches may be passed in on the command line, and some may not;
63 you could even get a "-" without its letter, if you're not careful.
64 You probably want to make sure that all your switches fall either
65 before or after that 32-character boundary. Most switches don't
66 actually care if they're processed redundantly, but getting a "-"
67 instead of a complete switch could cause Perl to try to execute
68 standard input instead of your program. And a partial B<-I> switch
69 could also cause odd results.
71 Some switches do care if they are processed twice, for instance
72 combinations of B<-l> and B<-0>. Either put all the switches after
73 the 32-character boundary (if applicable), or replace the use of
74 B<-0>I<digits> by C<BEGIN{ $/ = "\0digits"; }>.
76 Parsing of the C<#!> switches starts wherever "perl" is mentioned in the line.
77 The sequences "-*" and "- " are specifically ignored so that you could,
78 if you were so inclined, say
82 eval 'exec perl -x -wS $0 ${1+"$@"}'
85 to let Perl see the B<-p> switch.
87 A similar trick involves the I<env> program, if you have it.
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, perl5.14.1, you should place
94 that directly in the C<#!> line's path.
96 If the C<#!> line does not contain the word "perl" nor the word "indir",
97 the program named after the C<#!> is executed instead of the Perl
98 interpreter. This is slightly bizarre, but it helps people on machines
99 that don't do C<#!>, because they can tell a program that their SHELL is
100 F</usr/bin/perl>, and Perl will then dispatch the program to the correct
101 interpreter for them.
103 After locating your program, Perl compiles the entire program to an
104 internal form. If there are any compilation errors, execution of the
105 program is not attempted. (This is unlike the typical shell script,
106 which might run part-way through before finding a syntax error.)
108 If the program is syntactically correct, it is executed. If the program
109 runs off the end without hitting an exit() or die() operator, an implicit
110 C<exit(0)> is provided to indicate successful completion.
112 =head2 #! and quoting on non-Unix systems
115 Unix's C<#!> technique can be simulated on other systems:
123 extproc perl -S -your_switches
125 as the first line in C<*.cmd> file (B<-S> due to a bug in cmd.exe's
130 Create a batch file to run your program, and codify it in
131 C<ALTERNATE_SHEBANG> (see the F<dosish.h> file in the source
132 distribution for more information).
136 The Win95/NT installation, when using the ActiveState installer for Perl,
137 will modify the Registry to associate the F<.pl> extension with the perl
138 interpreter. If you install Perl by other means (including building from
139 the sources), you may have to modify the Registry yourself. Note that
140 this means you can no longer tell the difference between an executable
141 Perl program and a Perl library file.
147 $ perl -mysw 'f$env("procedure")' 'p1' 'p2' 'p3' 'p4' 'p5' 'p6' 'p7' 'p8' !
148 $ exit++ + ++$status != 0 and $exit = $status = undef;
150 at the top of your program, where B<-mysw> are any command line switches you
151 want to pass to Perl. You can now invoke the program directly, by saying
152 C<perl program>, or as a DCL procedure, by saying C<@program> (or implicitly
153 via F<DCL$PATH> by just using the name of the program).
155 This incantation is a bit much to remember, but Perl will display it for
156 you if you say C<perl "-V:startperl">.
160 Command-interpreters on non-Unix systems have rather different ideas
161 on quoting than Unix shells. You'll need to learn the special
162 characters in your command-interpreter (C<*>, C<\> and C<"> are
163 common) and how to protect whitespace and these characters to run
164 one-liners (see L<-e|/-e commandline> below).
166 On some systems, you may have to change single-quotes to double ones,
167 which you must I<not> do on Unix or Plan 9 systems. You might also
168 have to change a single % to a %%.
173 perl -e 'print "Hello world\n"'
176 perl -e "print \"Hello world\n\""
179 perl -e "print ""Hello world\n"""
181 The problem is that none of this is reliable: it depends on the
182 command and it is entirely possible neither works. If I<4DOS> were
183 the command shell, this would probably work better:
185 perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""
187 B<CMD.EXE> in Windows NT slipped a lot of standard Unix functionality in
188 when nobody was looking, but just try to find documentation for its
191 There is no general solution to all of this. It's just a mess.
193 =head2 Location of Perl
194 X<perl, location of interpreter>
196 It may seem obvious to say, but Perl is useful only when users can
197 easily find it. When possible, it's good for both F</usr/bin/perl>
198 and F</usr/local/bin/perl> to be symlinks to the actual binary. If
199 that can't be done, system administrators are strongly encouraged
200 to put (symlinks to) perl and its accompanying utilities into a
201 directory typically found along a user's PATH, or in some other
202 obvious and convenient place.
204 In this documentation, C<#!/usr/bin/perl> on the first line of the program
205 will stand in for whatever method works on your system. You are
206 advised to use a specific path if you care about a specific version.
208 #!/usr/local/bin/perl5.14
210 or if you just want to be running at least version, place a statement
211 like this at the top of your program:
215 =head2 Command Switches
216 X<perl, command switches> X<command switches>
218 As with all standard commands, a single-character switch may be
219 clustered with the following switch, if any.
221 #!/usr/bin/perl -spi.orig # same as -s -p -i.orig
223 A C<--> signals the end of options and disables further option processing. Any
224 arguments after the C<--> are treated as filenames and arguments.
230 =item B<-0>[I<octal/hexadecimal>]
233 specifies the input record separator (C<$/>) as an octal or
234 hexadecimal number. If there are no digits, the null character is the
235 separator. Other switches may precede or follow the digits. For
236 example, if you have a version of I<find> which can print filenames
237 terminated by the null character, you can say this:
239 find . -name '*.orig' -print0 | perl -n0e unlink
241 The special value 00 will cause Perl to slurp files in paragraph mode.
242 Any value 0400 or above will cause Perl to slurp files whole, but by convention
243 the value 0777 is the one normally used for this purpose.
245 You can also specify the separator character using hexadecimal notation:
246 B<-0xI<HHH...>>, where the C<I<H>> are valid hexadecimal digits. Unlike
247 the octal form, this one may be used to specify any Unicode character, even
248 those beyond 0xFF. So if you I<really> want a record separator of 0777,
249 specify it as B<-0x1FF>. (This means that you cannot use the B<-x> option
250 with a directory name that consists of hexadecimal digits, or else Perl
251 will think you have specified a hex number to B<-0>.)
256 turns on autosplit mode when used with a B<-n> or B<-p>. An implicit
257 split command to the @F array is done as the first thing inside the
258 implicit while loop produced by the B<-n> or B<-p>.
260 perl -ane 'print pop(@F), "\n";'
269 An alternate delimiter may be specified using B<-F>.
271 B<-a> implicitly sets B<-n>.
273 =item B<-C [I<number/list>]>
276 The B<-C> flag controls some of the Perl Unicode features.
278 As of 5.8.1, the B<-C> can be followed either by a number or a list
279 of option letters. The letters, their numeric values, and effects
280 are as follows; listing the letters is equal to summing the numbers.
282 I 1 STDIN is assumed to be in UTF-8
283 O 2 STDOUT will be in UTF-8
284 E 4 STDERR will be in UTF-8
286 i 8 UTF-8 is the default PerlIO layer for input streams
287 o 16 UTF-8 is the default PerlIO layer for output streams
289 A 32 the @ARGV elements are expected to be strings encoded
291 L 64 normally the "IOEioA" are unconditional, the L makes
292 them conditional on the locale environment variables
293 (the LC_ALL, LC_CTYPE, and LANG, in the order of
294 decreasing precedence) -- if the variables indicate
295 UTF-8, then the selected "IOEioA" are in effect
296 a 256 Set ${^UTF8CACHE} to -1, to run the UTF-8 caching
297 code in debugging mode.
299 =for documenting_the_underdocumented
300 perl.h gives W/128 as PERL_UNICODE_WIDESYSCALLS "/* for Sarathy */"
303 perltodo mentions Unicode in %ENV and filenames. I guess that these will be
304 options e and f (or F).
306 For example, B<-COE> and B<-C6> will both turn on UTF-8-ness on both
307 STDOUT and STDERR. Repeating letters is just redundant, not cumulative
310 The C<io> options mean that any subsequent open() (or similar I/O
311 operations) in the current file scope will have the C<:utf8> PerlIO layer
312 implicitly applied to them, in other words, UTF-8 is expected from any
313 input stream, and UTF-8 is produced to any output stream. This is just
314 the default, with explicit layers in open() and with binmode() one can
315 manipulate streams as usual.
317 B<-C> on its own (not followed by any number or option list), or the
318 empty string C<""> for the C<PERL_UNICODE> environment variable, has the
319 same effect as B<-CSDL>. In other words, the standard I/O handles and
320 the default C<open()> layer are UTF-8-fied I<but> only if the locale
321 environment variables indicate a UTF-8 locale. This behaviour follows
322 the I<implicit> (and problematic) UTF-8 behaviour of Perl 5.8.0.
323 (See L<perl581delta/UTF-8 no longer default under UTF-8 locales>.)
325 You can use B<-C0> (or C<"0"> for C<PERL_UNICODE>) to explicitly
326 disable all the above Unicode features.
328 The read-only magic variable C<${^UNICODE}> reflects the numeric value
329 of this setting. This variable is set during Perl startup and is
330 thereafter read-only. If you want runtime effects, use the three-arg
331 open() (see L<perlfunc/open>), the two-arg binmode() (see L<perlfunc/binmode>),
332 and the C<open> pragma (see L<open>).
334 (In Perls earlier than 5.8.1 the B<-C> switch was a Win32-only switch
335 that enabled the use of Unicode-aware "wide system call" Win32 APIs.
336 This feature was practically unused, however, and the command line
337 switch was therefore "recycled".)
339 B<Note:> Since perl 5.10.1, if the B<-C> option is used on the C<#!> line,
340 it must be specified on the command line as well, since the standard streams
341 are already set up at this point in the execution of the perl interpreter.
342 You can also use binmode() to set the encoding of an I/O stream.
347 causes Perl to check the syntax of the program and then exit without
348 executing it. Actually, it I<will> execute any C<BEGIN>, C<UNITCHECK>,
349 or C<CHECK> blocks and any C<use> statements: these are considered as
350 occurring outside the execution of your program. C<INIT> and C<END>
351 blocks, however, will be skipped.
358 runs the program under the Perl debugger. See L<perldebug>.
359 If B<t> is specified, it indicates to the debugger that threads
360 will be used in the code being debugged.
362 =item B<-d:>I<MOD[=bar,baz]>
365 =item B<-dt:>I<MOD[=bar,baz]>
367 runs the program under the control of a debugging, profiling, or tracing
368 module installed as C<Devel::I<MOD>>. E.g., B<-d:DProf> executes the
369 program using the C<Devel::DProf> profiler. As with the B<-M> flag, options
370 may be passed to the C<Devel::I<MOD>> package where they will be received
371 and interpreted by the C<Devel::I<MOD>::import> routine. Again, like B<-M>,
372 use -B<-d:-I<MOD>> to call C<Devel::I<MOD>::unimport> instead of import. The
373 comma-separated list of options must follow a C<=> character. If B<t> is
374 specified, it indicates to the debugger that threads will be used in the
375 code being debugged. See L<perldebug>.
377 =item B<-D>I<letters>
378 X<-D> X<DEBUGGING> X<-DDEBUGGING>
382 sets debugging flags. This switch is enabled only if your perl binary has
383 been built with debugging enabled: normal production perls won't have
386 For example, to watch how perl executes your program, use B<-Dtls>.
387 Another nice value is B<-Dx>, which lists your compiled syntax tree, and
388 B<-Dr> displays compiled regular expressions; the format of the output is
389 explained in L<perldebguts>.
391 As an alternative, specify a number instead of list of letters (e.g.,
392 B<-D14> is equivalent to B<-Dtls>):
394 1 p Tokenizing and parsing (with v, displays parse
396 2 s Stack snapshots (with v, displays all stacks)
397 4 l Context (loop) stack processing
399 16 o Method and overloading resolution
400 32 c String/numeric conversions
401 64 P Print profiling info, source file input state
402 128 m Memory and SV allocation
403 256 f Format processing
404 512 r Regular expression parsing and execution
405 1024 x Syntax tree dump
406 2048 u Tainting checks
407 4096 U Unofficial, User hacking (reserved for private,
409 16384 X Scratchpad allocation
411 65536 S Op slab allocation
413 262144 R Include reference counts of dumped variables
415 524288 J show s,t,P-debug (don't Jump over) on opcodes within
417 1048576 v Verbose: use in conjunction with other flags
418 2097152 C Copy On Write
419 4194304 A Consistency checks on internal structures
420 8388608 q quiet - currently only suppresses the "EXECUTING"
422 16777216 M trace smart match resolution
423 33554432 B dump suBroutine definitions, including special
425 67108864 L trace Locale-related info; what gets output is very
427 134217728 i trace PerlIO layer processing. Set PERLIO_DEBUG to
428 the filename to trace to.
430 All these flags require B<-DDEBUGGING> when you compile the Perl
431 executable (but see C<:opd> in L<Devel::Peek> or L<re/'debug' mode>
432 which may change this).
433 See the F<INSTALL> file in the Perl source distribution
436 If you're just trying to get a print out of each line of Perl code
437 as it executes, the way that C<sh -x> provides for shell scripts,
438 you can't use Perl's B<-D> switch. Instead do this
440 # If you have "env" utility
441 env PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
443 # Bourne shell syntax
444 $ PERLDB_OPTS="NonStop=1 AutoTrace=1 frame=2" perl -dS program
447 % (setenv PERLDB_OPTS "NonStop=1 AutoTrace=1 frame=2"; perl -dS program)
449 See L<perldebug> for details and variations.
451 =item B<-e> I<commandline>
454 may be used to enter one line of program. If B<-e> is given, Perl
455 will not look for a filename in the argument list. Multiple B<-e>
456 commands may be given to build up a multi-line script. Make sure
457 to use semicolons where you would in a normal program.
459 =item B<-E> I<commandline>
462 behaves just like B<-e>, except that it implicitly enables all
463 optional features (in the main compilation unit). See L<feature>.
466 X<-f> X<sitecustomize> X<sitecustomize.pl>
468 Disable executing F<$Config{sitelib}/sitecustomize.pl> at startup.
470 Perl can be built so that it by default will try to execute
471 F<$Config{sitelib}/sitecustomize.pl> at startup (in a BEGIN block).
472 This is a hook that allows the sysadmin to customize how Perl behaves.
473 It can for instance be used to add entries to the @INC array to make Perl
474 find modules in non-standard locations.
476 Perl actually inserts the following code:
479 do { local $!; -f "$Config{sitelib}/sitecustomize.pl"; }
480 && do "$Config{sitelib}/sitecustomize.pl";
483 Since it is an actual C<do> (not a C<require>), F<sitecustomize.pl>
484 doesn't need to return a true value. The code is run in package C<main>,
485 in its own lexical scope. However, if the script dies, C<$@> will not
488 The value of C<$Config{sitelib}> is also determined in C code and not
489 read from C<Config.pm>, which is not loaded.
491 The code is executed I<very> early. For example, any changes made to
492 C<@INC> will show up in the output of `perl -V`. Of course, C<END>
493 blocks will be likewise executed very late.
495 To determine at runtime if this capability has been compiled in your
496 perl, you can check the value of C<$Config{usesitecustomize}>.
498 =item B<-F>I<pattern>
501 specifies the pattern to split on for B<-a>. The pattern may be
502 surrounded by C<//>, C<"">, or C<''>, otherwise it will be put in single
503 quotes. You can't use literal whitespace or NUL characters in the pattern.
505 B<-F> implicitly sets both B<-a> and B<-n>.
510 prints a summary of the options.
512 =item B<-i>[I<extension>]
515 specifies that files processed by the C<E<lt>E<gt>> construct are to be
516 edited in-place. It does this by renaming the input file, opening the
517 output file by the original name, and selecting that output file as the
518 default for print() statements. The extension, if supplied, is used to
519 modify the name of the old file to make a backup copy, following these
522 If no extension is supplied, and your system supports it, the original
523 I<file> is kept open without a name while the output is redirected to
524 a new file with the original I<filename>. When perl exits, cleanly or not,
525 the original I<file> is unlinked.
527 If the extension doesn't contain a C<*>, then it is appended to the
528 end of the current filename as a suffix. If the extension does
529 contain one or more C<*> characters, then each C<*> is replaced
530 with the current filename. In Perl terms, you could think of this
533 ($backup = $extension) =~ s/\*/$file_name/g;
535 This allows you to add a prefix to the backup file, instead of (or in
536 addition to) a suffix:
538 $ perl -pi'orig_*' -e 's/bar/baz/' fileA # backup to
541 Or even to place backup copies of the original files into another
542 directory (provided the directory already exists):
544 $ perl -pi'old/*.orig' -e 's/bar/baz/' fileA # backup to
547 These sets of one-liners are equivalent:
549 $ perl -pi -e 's/bar/baz/' fileA # overwrite current file
550 $ perl -pi'*' -e 's/bar/baz/' fileA # overwrite current file
552 $ perl -pi'.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
553 $ perl -pi'*.orig' -e 's/bar/baz/' fileA # backup to 'fileA.orig'
555 From the shell, saying
557 $ perl -p -i.orig -e "s/foo/bar/; ... "
559 is the same as using the program:
561 #!/usr/bin/perl -pi.orig
564 which is equivalent to
567 $extension = '.orig';
569 if ($ARGV ne $oldargv) {
570 if ($extension !~ /\*/) {
571 $backup = $ARGV . $extension;
574 ($backup = $extension) =~ s/\*/$ARGV/g;
576 rename($ARGV, $backup);
577 open(ARGVOUT, ">$ARGV");
584 print; # this prints to original filename
588 except that the B<-i> form doesn't need to compare $ARGV to $oldargv to
589 know when the filename has changed. It does, however, use ARGVOUT for
590 the selected filehandle. Note that STDOUT is restored as the default
591 output filehandle after the loop.
593 As shown above, Perl creates the backup file whether or not any output
594 is actually changed. So this is just a fancy way to copy files:
596 $ perl -p -i'/some/file/path/*' -e 1 file1 file2 file3...
598 $ perl -p -i'.orig' -e 1 file1 file2 file3...
600 You can use C<eof> without parentheses to locate the end of each input
601 file, in case you want to append to each file, or reset line numbering
602 (see example in L<perlfunc/eof>).
604 If, for a given file, Perl is unable to create the backup file as
605 specified in the extension then it will skip that file and continue on
606 with the next one (if it exists).
608 For a discussion of issues surrounding file permissions and B<-i>, see
609 L<perlfaq5/Why does Perl let me delete read-only files? Why does -i clobber
610 protected files? Isn't this a bug in Perl?>.
612 You cannot use B<-i> to create directories or to strip extensions from
615 Perl does not expand C<~> in filenames, which is good, since some
616 folks use it for their backup files:
618 $ perl -pi~ -e 's/foo/bar/' file1 file2 file3...
620 Note that because B<-i> renames or deletes the original file before
621 creating a new file of the same name, Unix-style soft and hard links will
624 Finally, the B<-i> switch does not impede execution when no
625 files are given on the command line. In this case, no backup is made
626 (the original file cannot, of course, be determined) and processing
627 proceeds from STDIN to STDOUT as might be expected.
629 =item B<-I>I<directory>
632 Directories specified by B<-I> are prepended to the search path for
635 =item B<-l>[I<octnum>]
638 enables automatic line-ending processing. It has two separate
639 effects. First, it automatically chomps C<$/> (the input record
640 separator) when used with B<-n> or B<-p>. Second, it assigns C<$\>
641 (the output record separator) to have the value of I<octnum> so
642 that any print statements will have that separator added back on.
643 If I<octnum> is omitted, sets C<$\> to the current value of
644 C<$/>. For instance, to trim lines to 80 columns:
646 perl -lpe 'substr($_, 80) = ""'
648 Note that the assignment C<$\ = $/> is done when the switch is processed,
649 so the input record separator can be different than the output record
650 separator if the B<-l> switch is followed by a B<-0> switch:
652 gnufind / -print0 | perl -ln0e 'print "found $_" if -p'
654 This sets C<$\> to newline and then sets C<$/> to the null character.
656 =item B<-m>[B<->]I<module>
659 =item B<-M>[B<->]I<module>
661 =item B<-M>[B<->]I<'module ...'>
663 =item B<-[mM]>[B<->]I<module=arg[,arg]...>
665 B<-m>I<module> executes C<use> I<module> C<();> before executing your
666 program. This loads the module, but does not call its C<import> method,
667 so does not import subroutines and does not give effect to a pragma.
669 B<-M>I<module> executes C<use> I<module> C<;> before executing your
670 program. This loads the module and calls its C<import> method, causing
671 the module to have its default effect, typically importing subroutines
672 or giving effect to a pragma.
673 You can use quotes to add extra code after the module name,
674 e.g., C<'-MI<MODULE> qw(foo bar)'>.
676 If the first character after the B<-M> or B<-m> is a dash (B<->)
677 then the 'use' is replaced with 'no'.
678 This makes no difference for B<-m>.
680 A little builtin syntactic sugar means you can also say
681 B<-mI<MODULE>=foo,bar> or B<-MI<MODULE>=foo,bar> as a shortcut for
682 B<'-MI<MODULE> qw(foo bar)'>. This avoids the need to use quotes when
683 importing symbols. The actual code generated by B<-MI<MODULE>=foo,bar> is
684 C<use module split(/,/,q{foo,bar})>. Note that the C<=> form
685 removes the distinction between B<-m> and B<-M>; that is,
686 B<-mI<MODULE>=foo,bar> is the same as B<-MI<MODULE>=foo,bar>.
688 A consequence of the C<split> formulation
689 is that B<-MI<MODULE>=number> never does a version check,
690 unless C<I<MODULE>::import()> itself is set up to do a version check, which
691 could happen for example if I<MODULE> inherits from L<Exporter>.
696 causes Perl to assume the following loop around your program, which
697 makes it iterate over filename arguments somewhat like I<sed -n> or
702 ... # your program goes here
705 Note that the lines are not printed by default. See L</-p> to have
706 lines printed. If a file named by an argument cannot be opened for
707 some reason, Perl warns you about it and moves on to the next file.
709 Also note that C<< <> >> passes command line arguments to
710 L<perlfunc/open>, which doesn't necessarily interpret them as file names.
711 See L<perlop> for possible security implications.
713 Here is an efficient way to delete all files that haven't been modified for
716 find . -mtime +7 -print | perl -nle unlink
718 This is faster than using the B<-exec> switch of I<find> because you don't
719 have to start a process on every filename found (but it's not faster
720 than using the B<-delete> switch available in newer versions of I<find>.
721 It does suffer from the bug of mishandling newlines in pathnames, which
722 you can fix if you follow the example under B<-0>.
724 C<BEGIN> and C<END> blocks may be used to capture control before or after
725 the implicit program loop, just as in I<awk>.
730 causes Perl to assume the following loop around your program, which
731 makes it iterate over filename arguments somewhat like I<sed>:
736 ... # your program goes here
738 print or die "-p destination: $!\n";
741 If a file named by an argument cannot be opened for some reason, Perl
742 warns you about it, and moves on to the next file. Note that the
743 lines are printed automatically. An error occurring during printing is
744 treated as fatal. To suppress printing use the B<-n> switch. A B<-p>
745 overrides a B<-n> switch.
747 C<BEGIN> and C<END> blocks may be used to capture control before or after
748 the implicit loop, just as in I<awk>.
753 enables rudimentary switch parsing for switches on the command
754 line after the program name but before any filename arguments (or before
755 an argument of B<-->). Any switch found there is removed from @ARGV and sets the
756 corresponding variable in the Perl program. The following program
757 prints "1" if the program is invoked with a B<-xyz> switch, and "abc"
758 if it is invoked with B<-xyz=abc>.
761 if ($xyz) { print "$xyz\n" }
763 Do note that a switch like B<--help> creates the variable C<${-help}>, which is
764 not compliant with C<use strict "refs">. Also, when using this option on a
765 script with warnings enabled you may get a lot of spurious "used only once"
771 makes Perl use the PATH environment variable to search for the
772 program unless the name of the program contains path separators.
774 On some platforms, this also makes Perl append suffixes to the
775 filename while searching for it. For example, on Win32 platforms,
776 the ".bat" and ".cmd" suffixes are appended if a lookup for the
777 original name fails, and if the name does not already end in one
778 of those suffixes. If your Perl was compiled with C<DEBUGGING> turned
779 on, using the B<-Dp> switch to Perl shows how the search progresses.
781 Typically this is used to emulate C<#!> startup on platforms that don't
782 support C<#!>. It's also convenient when debugging a script that uses C<#!>,
783 and is thus normally found by the shell's $PATH search mechanism.
785 This example works on many platforms that have a shell compatible with
789 eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
790 if $running_under_some_shell;
792 The system ignores the first line and feeds the program to F</bin/sh>,
793 which proceeds to try to execute the Perl program as a shell script.
794 The shell executes the second line as a normal shell command, and thus
795 starts up the Perl interpreter. On some systems $0 doesn't always
796 contain the full pathname, so the B<-S> tells Perl to search for the
797 program if necessary. After Perl locates the program, it parses the
798 lines and ignores them because the variable $running_under_some_shell
799 is never true. If the program will be interpreted by csh, you will need
800 to replace C<${1+"$@"}> with C<$*>, even though that doesn't understand
801 embedded spaces (and such) in the argument list. To start up I<sh> rather
802 than I<csh>, some systems may have to replace the C<#!> line with a line
803 containing just a colon, which will be politely ignored by Perl. Other
804 systems can't control that, and need a totally devious construct that
805 will work under any of I<csh>, I<sh>, or Perl, such as the following:
807 eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
808 & eval 'exec /usr/bin/perl -wS $0 $argv:q'
809 if $running_under_some_shell;
811 If the filename supplied contains directory separators (and so is an
812 absolute or relative pathname), and if that file is not found,
813 platforms that append file extensions will do so and try to look
814 for the file with those extensions added, one by one.
816 On DOS-like platforms, if the program does not contain directory
817 separators, it will first be searched for in the current directory
818 before being searched for on the PATH. On Unix platforms, the
819 program will be searched for strictly on the PATH.
824 Like B<-T>, but taint checks will issue warnings rather than fatal
825 errors. These warnings can now be controlled normally with C<no warnings
828 B<Note: This is not a substitute for C<-T>!> This is meant to be
829 used I<only> as a temporary development aid while securing legacy code:
830 for real production code and for new secure code written from scratch,
831 always use the real B<-T>.
836 turns on "taint" so you can test them. Ordinarily
837 these checks are done only when running setuid or setgid. It's a
838 good idea to turn them on explicitly for programs that run on behalf
839 of someone else whom you might not necessarily trust, such as CGI
840 programs or any internet servers you might write in Perl. See
841 L<perlsec> for details. For security reasons, this option must be
842 seen by Perl quite early; usually this means it must appear early
843 on the command line or in the C<#!> line for systems which support
849 This switch causes Perl to dump core after compiling your
850 program. You can then in theory take this core dump and turn it
851 into an executable file by using the I<undump> program (not supplied).
852 This speeds startup at the expense of some disk space (which you
853 can minimize by stripping the executable). (Still, a "hello world"
854 executable comes out to about 200K on my machine.) If you want to
855 execute a portion of your program before dumping, use the dump()
856 operator instead. Note: availability of I<undump> is platform
857 specific and may not be available for a specific port of Perl.
862 allows Perl to do unsafe operations. Currently the only "unsafe"
863 operations are attempting to unlink directories while running as superuser
864 and running setuid programs with fatal taint checks turned into warnings.
865 Note that warnings must be enabled along with this option to actually
866 I<generate> the taint-check warnings.
871 prints the version and patchlevel of your perl executable.
876 prints summary of the major perl configuration values and the current
879 =item B<-V:>I<configvar>
881 Prints to STDOUT the value of the named configuration variable(s),
882 with multiples when your C<I<configvar>> argument looks like a regex (has
883 non-letters). For example:
886 libc='/lib/libc-2.2.4.so';
888 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
889 libc='/lib/libc-2.2.4.so';
891 libpth='/usr/local/lib /lib /usr/lib';
892 libs='-lnsl -lgdbm -ldb -ldl -lm -lcrypt -lutil -lc';
894 libc='/lib/libc-2.2.4.so';
898 Additionally, extra colons can be used to control formatting. A
899 trailing colon suppresses the linefeed and terminator ";", allowing
900 you to embed queries into shell commands. (mnemonic: PATH separator
903 $ echo "compression-vars: " `perl -V:z.*: ` " are here !"
904 compression-vars: zcat='' zip='zip' are here !
906 A leading colon removes the "name=" part of the response, this allows
907 you to map to the name you need. (mnemonic: empty label)
909 $ echo "goodvfork="`./perl -Ilib -V::usevfork`
912 Leading and trailing colons can be used together if you need
913 positional parameter values without the names. Note that in the case
914 below, the C<PERL_API> params are returned in alphabetical order.
916 $ echo building_on `perl -V::osname: -V::PERL_API_.*:` now
917 building_on 'linux' '5' '1' '9' now
922 prints warnings about dubious constructs, such as variable names
923 mentioned only once and scalar variables used
924 before being set; redefined subroutines; references to undefined
925 filehandles; filehandles opened read-only that you are attempting
926 to write on; values used as a number that don't I<look> like numbers;
927 using an array as though it were a scalar; if your subroutines
928 recurse more than 100 deep; and innumerable other things.
930 This switch really just enables the global C<$^W> variable; normally,
931 the lexically scoped C<use warnings> pragma is preferred. You
932 can disable or promote into fatal errors specific warnings using
933 C<__WARN__> hooks, as described in L<perlvar> and L<perlfunc/warn>.
934 See also L<perldiag> and L<perltrap>. A fine-grained warning
935 facility is also available if you want to manipulate entire classes
936 of warnings; see L<warnings>.
941 Enables all warnings regardless of C<no warnings> or C<$^W>.
947 Disables all warnings regardless of C<use warnings> or C<$^W>.
953 =item B<-x>I<directory>
955 tells Perl that the program is embedded in a larger chunk of unrelated
956 text, such as in a mail message. Leading garbage will be
957 discarded until the first line that starts with C<#!> and contains the
958 string "perl". Any meaningful switches on that line will be applied.
960 All references to line numbers by the program (warnings, errors, ...)
961 will treat the C<#!> line as the first line.
962 Thus a warning on the 2nd line of the program, which is on the 100th
963 line in the file will be reported as line 2, not as line 100.
964 This can be overridden by using the C<#line> directive.
965 (See L<perlsyn/"Plain Old Comments (Not!)">)
967 If a directory name is specified, Perl will switch to that directory
968 before running the program. The B<-x> switch controls only the
969 disposal of leading garbage. The program must be terminated with
970 C<__END__> if there is trailing garbage to be ignored; the program
971 can process any or all of the trailing garbage via the C<DATA> filehandle
974 The directory, if specified, must appear immediately following the B<-x>
975 with no intervening whitespace.
980 X<perl, environment variables>
987 Used if C<chdir> has no argument.
992 Used if C<chdir> has no argument and HOME is not set.
997 Used in executing subprocesses, and in finding the program if B<-S> is
1003 A list of directories in which to look for Perl library files before
1004 looking in the standard library.
1005 Any architecture-specific and version-specific directories,
1006 such as F<version/archname/>, F<version/>, or F<archname/> under the
1007 specified locations are automatically included if they exist, with this
1008 lookup done at interpreter startup time. In addition, any directories
1009 matching the entries in C<$Config{inc_version_list}> are added.
1010 (These typically would be for older compatible perl versions installed
1011 in the same directory tree.)
1013 If PERL5LIB is not defined, PERLLIB is used. Directories are separated
1014 (like in PATH) by a colon on Unixish platforms and by a semicolon on
1015 Windows (the proper path separator being given by the command C<perl
1018 When running taint checks, either because the program was running setuid or
1019 setgid, or the B<-T> or B<-t> switch was specified, neither PERL5LIB nor
1020 PERLLIB is consulted. The program should instead say:
1022 use lib "/my/directory";
1027 Command-line options (switches). Switches in this variable are treated
1028 as if they were on every Perl command line. Only the B<-[CDIMUdmtwW]>
1029 switches are allowed. When running taint checks (either because the
1030 program was running setuid or setgid, or because the B<-T> or B<-t>
1031 switch was used), this variable is ignored. If PERL5OPT begins with
1032 B<-T>, tainting will be enabled and subsequent options ignored. If
1033 PERL5OPT begins with B<-t>, tainting will be enabled, a writable dot
1034 removed from @INC, and subsequent options honored.
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 affect Perl's IO.
1042 It is conventional to start layer names with a colon (for example, C<:perlio>) to
1043 emphasize 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.
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.
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 C<:encoding()>) need
1053 IO in order to load them! See L<"open pragma"|open> for how to add external
1054 encodings as defaults.
1056 Layers it makes sense to include in the PERLIO environment
1057 variable are briefly summarized below. For more details see L<PerlIO>.
1064 A pseudolayer that turns the C<:utf8> flag I<off> 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>.
1071 A layer which does CRLF to C<"\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.)
1079 A layer that implements "reading" of files by using I<mmap>(2) to
1080 make an entire file appear in the process's address space, and then
1081 using that as PerlIO's "buffer".
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>.
1093 An experimental pseudolayer that removes the topmost layer.
1094 Use with the same care as is reserved for nitroglycerine.
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 translation. In particular, both CRLF
1102 translation and intuiting C<:utf8> from the locale are disabled.
1104 Unlike in 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.
1111 This layer provides a PerlIO interface by wrapping system's ANSI C "stdio"
1112 library calls. The layer provides both buffering and IO.
1113 Note that the C<:stdio> layer does I<not> do CRLF translation even if that
1114 is the platform's normal behaviour. You will need a C<:crlf> layer above it
1120 Low-level layer that calls C<read>, C<write>, C<lseek>, etc.
1125 A pseudolayer that enables a flag in 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. B<WARNING: It does not check for validity and as such
1128 should be handled with extreme caution for input, because security violations
1129 can occur with non-shortest UTF-8 encodings, etc.> Generally C<:encoding(UTF-8)> is
1130 the best option when reading UTF-8 encoded data.
1135 On Win32 platforms this I<experimental> layer uses native "handle" IO
1136 rather than a Unix-like numeric file descriptor layer. Known to be
1137 buggy in this release (5.14).
1141 The default set of layers should give acceptable results on all platforms
1143 For Unix platforms that will be the equivalent of "unix perlio" or "stdio".
1144 Configure is set up to prefer the "stdio" implementation if the system's library
1145 provides for fast access to the buffer; otherwise, it uses the "unix perlio"
1148 On Win32 the default in this release (5.14) is "unix crlf". Win32's "stdio"
1149 has a number of bugs/mis-features for Perl IO which are somewhat depending
1150 on the version and vendor of the C compiler. Using our own C<crlf> layer as
1151 the buffer avoids those issues and makes things more uniform. The C<crlf>
1152 layer provides CRLF conversion as well as buffering.
1154 This release (5.14) uses C<unix> as the bottom layer on Win32, and so still
1155 uses the C compiler's numeric file descriptor routines. There is an
1156 experimental native C<win32> layer, which is expected to be enhanced and
1157 should eventually become the default under Win32.
1159 The PERLIO environment variable is completely ignored when Perl
1160 is run in taint mode.
1165 If set to the name of a file or device when Perl is run with the
1166 B<-Di> command-line switch, the logging of certain operations of
1167 the PerlIO subsystem will be redirected to the specified file rather
1168 than going to stderr, which is the default. The file is opened in append
1169 mode. Typical uses are in Unix:
1171 % env PERLIO_DEBUG=/tmp/perlio.log perl -Di script ...
1173 and under Win32, the approximately equivalent:
1175 > set PERLIO_DEBUG=CON
1178 This functionality is disabled for setuid scripts, for scripts run
1179 with B<-T>, and for scripts run on a Perl built without C<-DDEBUGGING>
1185 A list of directories in which to look for Perl library
1186 files before looking in the standard library.
1187 If PERL5LIB is defined, PERLLIB is not used.
1189 The PERLLIB environment variable is completely ignored when Perl
1190 is run in taint mode.
1195 The command used to load the debugger code. The default is:
1197 BEGIN { require "perl5db.pl" }
1199 The PERL5DB environment variable is only used when Perl is started with
1200 a bare B<-d> switch.
1202 =item PERL5DB_THREADED
1205 If set to a true value, indicates to the debugger that the code being
1206 debugged uses threads.
1208 =item PERL5SHELL (specific to the Win32 port)
1211 On Win32 ports only, may be set to an alternative shell that Perl must use
1212 internally for executing "backtick" commands or system(). Default is
1213 C<cmd.exe /x/d/c> on WindowsNT and C<command.com /c> on Windows95. The
1214 value is considered space-separated. Precede any character that
1215 needs to be protected, like a space or backslash, with another backslash.
1217 Note that Perl doesn't use COMSPEC for this purpose because
1218 COMSPEC has a high degree of variability among users, leading to
1219 portability concerns. Besides, Perl can use a shell that may not be
1220 fit for interactive use, and setting COMSPEC to such a shell may
1221 interfere with the proper functioning of other programs (which usually
1222 look in COMSPEC to find a shell fit for interactive use).
1224 Before Perl 5.10.0 and 5.8.8, PERL5SHELL was not taint checked
1225 when running external commands. It is recommended that
1226 you explicitly set (or delete) C<$ENV{PERL5SHELL}> when running
1227 in taint mode under Windows.
1229 =item PERL_ALLOW_NON_IFS_LSP (specific to the Win32 port)
1230 X<PERL_ALLOW_NON_IFS_LSP>
1232 Set to 1 to allow the use of non-IFS compatible LSPs (Layered Service Providers).
1233 Perl normally searches for an IFS-compatible LSP because this is required
1234 for its emulation of Windows sockets as real filehandles. However, this may
1235 cause problems if you have a firewall such as I<McAfee Guardian>, which requires
1236 that all applications use its LSP but which is not IFS-compatible, because clearly
1237 Perl will normally avoid using such an LSP.
1239 Setting this environment variable to 1 means that Perl will simply use the
1240 first suitable LSP enumerated in the catalog, which keeps I<McAfee Guardian>
1241 happy--and in that particular case Perl still works too because I<McAfee
1242 Guardian>'s LSP actually plays other games which allow applications
1243 requiring IFS compatibility to work.
1245 =item PERL_DEBUG_MSTATS
1246 X<PERL_DEBUG_MSTATS>
1248 Relevant only if Perl is compiled with the C<malloc> included with the Perl
1249 distribution; that is, if C<perl -V:d_mymalloc> is "define".
1251 If set, this dumps out memory statistics after execution. If set
1252 to an integer greater than one, also dumps out memory statistics
1255 =item PERL_DESTRUCT_LEVEL
1256 X<PERL_DESTRUCT_LEVEL>
1258 Relevant only if your Perl executable was built with B<-DDEBUGGING>,
1259 this controls the behaviour of global destruction of objects and other
1260 references. See L<perlhacktips/PERL_DESTRUCT_LEVEL> for more information.
1262 =item PERL_DL_NONLAZY
1265 Set to C<"1"> to have Perl resolve I<all> undefined symbols when it loads
1266 a dynamic library. The default behaviour is to resolve symbols when
1267 they are used. Setting this variable is useful during testing of
1268 extensions, as it ensures that you get an error on misspelled function
1269 names even if the test suite doesn't call them.
1274 If using the C<use encoding> pragma without an explicit encoding name, the
1275 PERL_ENCODING environment variable is consulted for an encoding name.
1277 =item PERL_HASH_SEED
1280 (Since Perl 5.8.1, new semantics in Perl 5.18.0) Used to override
1281 the randomization of Perl's internal hash function. The value is expressed
1282 in hexadecimal, and may include a leading 0x. Truncated patterns
1283 are treated as though they are suffixed with sufficient 0's as required.
1285 If the option is provided, and C<PERL_PERTURB_KEYS> is NOT set, then
1286 a value of '0' implies C<PERL_PERTURB_KEYS=0> and any other value
1287 implies C<PERL_PERTURB_KEYS=2>.
1289 B<PLEASE NOTE: The hash seed is sensitive information>. Hashes are
1290 randomized to protect against local and remote attacks against Perl
1291 code. By manually setting a seed, this protection may be partially or
1294 See L<perlsec/"Algorithmic Complexity Attacks">, L</PERL_PERTURB_KEYS>, and
1295 L</PERL_HASH_SEED_DEBUG> for more information.
1297 =item PERL_PERTURB_KEYS
1298 X<PERL_PERTURB_KEYS>
1300 (Since Perl 5.18.0) Set to C<"0"> or C<"NO"> then traversing keys
1301 will be repeatable from run to run for the same PERL_HASH_SEED.
1302 Insertion into a hash will not change the order, except to provide
1303 for more space in the hash. When combined with setting PERL_HASH_SEED
1304 this mode is as close to pre 5.18 behavior as you can get.
1306 When set to C<"1"> or C<"RANDOM"> then traversing keys will be randomized.
1307 Every time a hash is inserted into the key order will change in a random
1308 fashion. The order may not be repeatable in a following program run
1309 even if the PERL_HASH_SEED has been specified. This is the default
1312 When set to C<"2"> or C<"DETERMINISTIC"> then inserting keys into a hash
1313 will cause the key order to change, but in a way that is repeatable
1314 from program run to program run.
1316 B<NOTE:> Use of this option is considered insecure, and is intended only
1317 for debugging non-deterministic behavior in Perl's hash function. Do
1318 not use it in production.
1320 See L<perlsec/"Algorithmic Complexity Attacks"> and L</PERL_HASH_SEED>
1321 and L</PERL_HASH_SEED_DEBUG> for more information. You can get and set the
1322 key traversal mask for a specific hash by using the C<hash_traversal_mask()>
1323 function from L<Hash::Util>.
1325 =item PERL_HASH_SEED_DEBUG
1326 X<PERL_HASH_SEED_DEBUG>
1328 (Since Perl 5.8.1.) Set to C<"1"> to display (to STDERR) information
1329 about the hash function, seed, and what type of key traversal
1330 randomization is in effect at the beginning of execution. This, combined
1331 with L</PERL_HASH_SEED> and L</PERL_PERTURB_KEYS> is intended to aid in
1332 debugging nondeterministic behaviour caused by hash randomization.
1334 B<Note> that any information about the hash function, especially the hash
1335 seed is B<sensitive information>: by knowing it, one can craft a denial-of-service
1336 attack against Perl code, even remotely; see L<perlsec/"Algorithmic Complexity Attacks">
1337 for more information. B<Do not disclose the hash seed> to people who
1338 don't need to know it. See also C<hash_seed()> and
1339 C<key_traversal_mask()> in L<Hash::Util>.
1341 An example output might be:
1343 HASH_FUNCTION = ONE_AT_A_TIME_HARD HASH_SEED = 0x652e9b9349a7a032 PERTURB_KEYS = 1 (RANDOM)
1348 If your Perl was configured with B<-Accflags=-DPERL_MEM_LOG>, setting
1349 the environment variable C<PERL_MEM_LOG> enables logging debug
1350 messages. The value has the form C<< <I<number>>[m][s][t] >>, where
1351 C<I<number>> is the file descriptor number you want to write to (2 is
1352 default), and the combination of letters specifies that you want
1353 information about (m)emory and/or (s)v, optionally with
1354 (t)imestamps. For example, C<PERL_MEM_LOG=1mst> logs all
1355 information to stdout. You can write to other opened file descriptors
1356 in a variety of ways:
1358 $ 3>foo3 PERL_MEM_LOG=3m perl ...
1360 =item PERL_ROOT (specific to the VMS port)
1363 A translation-concealed rooted logical name that contains Perl and the
1364 logical device for the @INC path on VMS only. Other logical names that
1365 affect Perl on VMS include PERLSHR, PERL_ENV_TABLES, and
1366 SYS$TIMEZONE_DIFFERENTIAL, but are optional and discussed further in
1367 L<perlvms> and in F<README.vms> in the Perl source distribution.
1372 Available in Perls 5.8.1 and later. If set to C<"unsafe">, the pre-Perl-5.8.0
1373 signal behaviour (which is immediate but unsafe) is restored. If set
1374 to C<safe>, then safe (but deferred) signals are used. See
1375 L<perlipc/"Deferred Signals (Safe Signals)">.
1380 Equivalent to the B<-C> command-line switch. Note that this is not
1381 a boolean variable. Setting this to C<"1"> is not the right way to
1382 "enable Unicode" (whatever that would mean). You can use C<"0"> to
1383 "disable Unicode", though (or alternatively unset PERL_UNICODE in
1384 your shell before starting Perl). See the description of the B<-C>
1385 switch for more information.
1387 =item PERL_USE_UNSAFE_INC
1388 X<PERL_USE_UNSAFE_INC>
1390 If perl has been configured to not have the current directory in
1391 L<C<@INC>|perlvar/@INC> by default, this variable can be set to C<"1">
1392 to reinstate it. It's primarily intended for use while building and
1393 testing modules that have not been updated to deal with "." not being in
1394 C<@INC> and should not be set in the environment for day-to-day use.
1396 =item SYS$LOGIN (specific to the VMS port)
1399 Used if chdir has no argument and HOME and LOGDIR are not set.
1401 =item PERL_INTERNAL_RAND_SEED
1402 X<PERL_INTERNAL_RAND_SEED>
1404 Set to a non-negative integer to seed the random number generator used
1405 internally by perl for a variety of purposes.
1407 Ignored if perl is run setuid or setgid. Used only for some limited
1408 startup randomization (hash keys) if C<-T> or C<-t> perl is started
1409 with tainting enabled.
1411 Perl may be built to ignore this variable.
1415 Perl also has environment variables that control how Perl handles data
1416 specific to particular natural languages; see L<perllocale>.
1418 Perl and its various modules and components, including its test frameworks,
1419 may sometimes make use of certain other environment variables. Some of
1420 these are specific to a particular platform. Please consult the
1421 appropriate module documentation and any documentation for your platform
1422 (like L<perlsolaris>, L<perllinux>, L<perlmacosx>, L<perlwin32>, etc) for
1423 variables peculiar to those specific situations.
1425 Perl makes all environment variables available to the program being
1426 executed, and passes these along to any child processes it starts.
1427 However, programs running setuid would do well to execute the following
1428 lines before doing anything else, just to keep people honest:
1430 $ENV{PATH} = "/bin:/usr/bin"; # or whatever you need
1431 $ENV{SHELL} = "/bin/sh" if exists $ENV{SHELL};
1432 delete @ENV{qw(IFS CDPATH ENV BASH_ENV)};