Mod_Perl is an optimisation

I presume you're talking about content delivery, and perhaps maybe not even more than using Apache/ModPerl::Registry to execute formerly CGI.pm-based Perl CGI programs.

There's a lot more to mod_perl than simply the response phase. For everything else, mod_perl is not an optimization, but something that is adding serious functionality. For example, I rewrote mod_usertrack in Perl so that it actually worked on my installation. I also have log handler that writes directly to DBI. Neither of those are an "optimization": they are essential functionality.