If you search for "thread leak" on perlmonks, you find the answer, the problem was discussed many times here. Shortly, on *nix perl threads leaking memory every time you creating a new thread. The bug was fixed somewhere around 5.13.9 if I remember correctly.

You should not create many threads, thread creation is rather expensive operation (on *nix it is more expensive than fork, I'm talking about perl threads of course). Solution is to use pool of threads.