php is hard to compare with Perl itself, for Perl is a general purpose
language. It makes much more sense to compare it with several server-side scriptings based on Perl, such as Embperl and HTML::Mason.
Embperl is page oriented, fast (written in C/XS), and added syntatic sugars for web development convenience.
HTML::Mason is written in pure perl, more site oriented, and offers many features which is hard to find on other templating systems such as dhandler, autohandler, caching, and output filtering.
Assuming that these are run under mod_perl, Mason is slower than Embperl, and I think Embperl performance is comparable to php's.
Raw mod_perl is faster than php. But I don't think that this speed differences is significant in most web apps.
An apache + mod_perl httpd size is large, 10-20M is typical after loading some required perl modules. So you may take this into account when you want to build an apache + mod_perl + mod_php + mod_?
php is appropriate for most web tasks, but if you want some esoteric functionality, or you want to get the best performance, you'd better use mod_perl.