Check out Apache::VMonitor, which "emulates the reporting functionalities of top(), mount(), df() and ifconfig() utilities".

The author, Stas Bekman, has also written a Perl wrapper around ApacheBench (ab). You might check that out, as well. It's called Apache::Benchmark, and it's not yet on CPAN.

You might also check out the Performance Tuning section of the mod_perl guide. And the Performance section of the Apache docs.