You need to find out where in perl the call occurs. You also need to rule out PDK as a factor. You could use Devel::Trace to help you, or sprinkle print statements in your code. You could also recompile Win32::GUI with debug flags turned on for additional hints... then write code to reproduce that error on demand, something you can send in bug report rt://Win32-GUI (or to PDK, if PDK helps cause error).