Some ideas (speculation without knowing what your Perl is doing)-

• Rewrite the Perl code in PHP and use it directly; they can translate pretty directly sometimes.
• Turn the Perl script into a daemon listening on a socket or a URI and have the PHP make the requests there. This is essentially the same solution as mod_perl but *might* be easier depending on the situation and your Perl chops. Search CPAN or here for "daemon" for modules and recommendations.
• mod_perl. Errto's registry suggestion is probably the easiest way to do this.
• Turn your script into fastcgi (FCGI). It is generally easier and more likely to be available/supported than mod_perl. They are roughly equivalent in terms of performance. mod_perl is much deeper but its apache hooks are rarely needed in plain web scripting.

Whatever you pick, you're in for a bit of work/learning. I'd personally go with the fastcgi solution. Good luck.