Free to wrong pool

I've been bitten by that as well when using Tk.It is an issue with reentrancy and/or thread safety.You might want to check this node out for more info Perl Tk and threads

You might also want to follow Corion's advise of sending a Window message back to the GUI thread since the call will be serialized in its message queue and thus won't break thread safety.

You might also run the app under Leakdiag and LDGraph which parses the data collected from Leakdiag and presents a nice diagram with the suspicious memory allocations so it might help you pinpointing the leak