With mod_perl, you have more control over the apache request,
and are able to do processing at almost any stage.
You also have the benefit of not having to fork and start
up a perl interpreter, as it is already loaded into memory.
This can give you a significant speed increase, at the cost
of a fatter apache process.
If you are looking at mod_perl, these links may help.
- mod_perl - The mod_perl site.
- Mason - A templating/web development system
As for speed, most benchmarks I've seen, mod_php was faster for small things (hello world type stuff - less overhead).
Of course, generally dynamic applications are constrained by databases and the like, rather than perl/php speed.
Update: To clarify above notes, with php (at least up to php4, I'm not all that familiar with 5), you have much more control over apache with mod_perl, than mod_php.
Furthermore, speed isn't really an issue. If mod_perl is too slow, odds are php is too slow as well, and you might start considering a standard C apache module.