In Perl, there is this f*ing ref count thing, which cause complex, non-threadsafe modules, to not be completely destroyed, due to them still wanting to use a variable.

You can google for "perl memory thread safety" and see the gory details. My advice, if you want to play it safe, use a reusable-thread model in your software design. You create threads once, then reuse them, avoiding the need to worry about threads-based memory gains to the perl interpreter. So see Reusable threads demo and OS memory reclamation with threads on linux for examples.