$mw->messageBox( ... ) surely overwrites $^E. The "overlapped" 'error' is often found in $^E, probably because some common, internal task that perl.exe performs generates that result.

Once you grab $^E's contents sooner, your next problem will likely be figuring out which method call is the one that is failing. But you should be able to get the real failure reason, which may be enough to point a finger somewhere.