Keep in mind that big data structures your code builds up aren't shared, since they are build up in the child processes. If you have big static data structures you could build them in a file PerlRequired from apache, at least then they become shared. Remember to destroy/free all big dynamic data structures.

There's a few tips on measuring memory usage in Measuring Code Memory Usage from the Practical mod_perl book, there's a chapter on mod_perl2 specifics too.