Ditto the comments on mod_perl documentation. You may want to check out Apache::VMonitor which lets you view an advanced scoreboard of system stats. You can edit this module to add your own stats too.

One thing to be careful about, if you are looking at memory and run more than one mod_perl child, they will be sharing memory efficiently but may seem on the scoreboard to be taking up extra space (it does say "shared" for that value though)

Requires GTop, Time::HiRes and Apache::Scoreboard.