You'd have to do benchmark tests to get the true results.
PERL could be faster because it's been around longer and probably has a few more efficient routines than PHP.
On the other hand, PHP is usually run as a module for the webserver, which means that the PHP binary doesn't have to be loaded from disk as with most PERL cases, which means it saves time and memory when being loaded.
On the other hand, running together with the webserver means there is always one webserver process for each PHP page, not so for PERL.
On the other hand, I could be really good at PERL and crap at PHP, so my PERL code is bound to be more efficient and thus faster.
On the other hand, I am good at PHP and slightly less good at PERL.
Did I mention I'm very modest?
All joking aside, if you are after some raw data processing thast doesn't need any web interfacing, you'd really have to just try it out.
If you do need web interfacing, PHP is just so much easier to make that any speed difference would have to be quite huge to be a problem.
