Sounds like the program is designed (either intentionally or accidentally) to need more memory as it runs. Either way, that's not related to the memory-releasing characteristics of perl. (You can't expect a process to release memory that it thinks it's still using.)
in reply to How to deal with the fact that Perl is not releasing memory
Do you think it should need extra memory as it runs ? If not, then there's probably a memory leak (unused memory not being freed for re-use by the process) that needs to be fixed.