One plausible explanation could be that even though perl actually garbage collects it, it's not released to the OS to be reclaimed just yet. If that is indeed the case then it seems like long running tasks/processes like mod_perl continually grow in size until that particular apache/mod_perl http process is killed/terminated.

Yeah thats how it works. If you read around you'll find it pretty thoroughly documented and discussed.

If I have an operation that I need to perform from a long running process thats going to use up a big chunk of memory that I want to be returned to the OS, I put the task in a job queue for a different, short running process to handle.

regards,