Well, you can look at how Apache::SizeLimit and Apache::GTopLimit do it, but this is not likely to help you find leaks. Perl allocates memory in chunks, and it grabs a bit ahead of what it's actually using. The instruction executed just before a large gain in memory is not necessarilly the one responsible for most of that memory getting used.

The best way to find leaks is one that Matts described on the mod_perl list: run a section of code over and over, and watch the process with top to see if it grows over time. If you don't have any leaks, it will level off after maybe 100 or so requests and not grow anymore. If you do have leaks it will never stop growing.

Matt tested his code by throwing an exception to jump out of his program right after the section of code he was testing, and he would keep moving the exception deeper into the code until he found the leaky part. Tedious, but effective.