There is only one reliable method of looking for leaks: keep pulling stuff out until the leak stops. It's a pain, but it's really the only way.

Don't forget that perl allocates memory in chunks at a time, so it may go over the leaking code a few times without growing. This unfortunately means that you can't just go line by line in the debugger and watch for growth. Some leaky code has to be run a few times before it grabs enough memory to show the leak.