You might be interested in OS memory reclamation with threads on linux. I'm no expert, but it seems that Perl uses some internal calculator, to determine when, and how much memory to free back to the system. It is clearly seen in the above node, where a memory-heavy thread is almost totally released back to the system, but with light-weight threads, it is held onto.

I was musing the other day, that it would be a neat feature to have a "forced demalloc" on threads, where you could specify an option to free all memory used by a thread once it's done, damn the refcount. I would like that option, as it would then be easy to reclaim memory just by putting it in a thread, and specifying "free_all". Possibly warnings may be issued, but another "no warnings:free_all" could be used.