When a Perl script is run from the command line, the shell invokes the Perl interpreter via the #!/bin/perl directive, which is the first line of the script (sometimes referred to as the shebang line). In scripts running under mod_cgi, you may use Perl switches as described in the perlrun manpage, such as -w, -T, or -d. Under the Apache::Registry handlers family, all switches except -w are ignored (and use of the -T switch triggers a warning). The support for -w was added for backward compatibility with mod_cgi.

Most command-line switches have special Perl variable equivalents that allow them to be set/unset in code. Consult the perlvar manpage for more details.

mod_perl provides its own equivalents to -w and -T in the form of configuration directives, as we'll discuss presently.

Finally, if you still need to set additional Perl startup flags, such as -d and -D, you can use the PERL5OPT environment variable. Switches in this variable are treated as if they were on every Perl command line. According to the perlrun manpage, only the -[DIMUdmw] switches are allowed.