As stated above, thread memory is returned to the process when the thread terminates, not the OS. The memory becomes a part of the free memory pool(s) that new program elements, both code and data will be allocated from as needed.

What that means, is that if any of your existing threads allocate scalars, or arrays or instantiate new instance of classes etc. etc., then the memory from terminated threads is recycled to provide it. If you start a new thread, then that memory will be used to provide for that new thread.

At the very extreme, if your threaded perl process terminated threads and never needed to allocate another bean of memory, and other processes on your system continued to call for more and more memory until the only "free memory" in the system was that freed from the terminated threads within your process, then the OS would swap that free memory to disk, and the other process would be allocated what it needed.

That called swapping--and everyone knows swapping is bad right?-- but the clever bit is that if your process never attempts to re-use that swapped out free (virtual) memory, then it will just stay on disk and so it's equivalent of real memory will be available to any and every other process in the system anyway. Swapping is only bad if it happens to memory that is in constant use and results in thrashing.

So, what is the "memory issue" you have with Perl's threads?