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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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

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

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

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

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

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

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

Because many operating systems permit anyone with read access to the Perl program file to make a copy of it, patch the copy, and then execute the copy, the security-conscious Perl programmer should take care to invoke the installed copy of perl, not the copy referenced by $^X. The following statements accomplish this goal, and produce a pathname that can be invoked as a command or referenced as a file.

use Config;
$secure_perl_path = $Config{perlpath};
if ($^O ne 'VMS')
   {$secure_perl_path .= $Config{_exe}
        unless $secure_perl_path =~ m/$Config{_exe}$/i;}
ISBN 9781906966027Perl Language Reference ManualSee the print edition