Perl may be done using that memory for the moment, but virtually all OS's don't reclaim it. However, if you were to rebuild another data-structure that needed gobs of memory, the same memory would be reused within your current process.
in reply to Not able to release memory
Update: I should mention that if you really need to force your OS to reclaim memory, you should start looking at using exec to spawn a new process while terminating the current one. Of course you may need to figure out a solution to maintain state, but that's another discussion.