An "attempt to free unreferenced scalar" basically means the perl interpreter got confused somewhere about the reference count of some object. This usually indicates a serious issue with either the interpreter itself or some of the XS/C based extensions you're using.

Given that you're using threads, it really could be either, but I would probably start by not using threads and see if the problem goes away. There really aren't many situations where threads are the best option (at least, not in perl).