Looks like a bug to me. If GD::Image is sending something to STDERR (like it seems to) then it should be manageable thru $SIG{__WARN__}. Try redirecting the STDERR and see what's happenning.

Update: Looks like actually XS Module stderr can't be easily catched, there's even a module for that: IO::CaptureOutput, hence that should provide you with the solution to your problem.