If you're not able to check out the code to add print statements, how will you fix any bugs that you find?

Otherwise I was going to suggest that, at the points in the module where you want the debugger to break, you add the line: $DB::single = 1 (That's like hitting 's' in the debugger right at that line.)

This might help with setting breakpoints in the module: you can change which file you're looking at/setting breakpoints in/etc. by using the debugger's 'f' command.