You can hook on any internal apache processing step, not just the final page generation (as with mod_php, mod_rails, etc). This allows you, for example, to use Perl code instead of Rewrite Rules, and still using Apache's fast to finally deliver static content. Moreover, you could theoretically provide the whole Apache configuration via Perl code.
Although that kind of stuff may sound complicated, it allows for great flexibility and I've been told that this actually simplifies things.
