But for a normal perl program, let's say a simple non-interactive script running from the command line as a seperate process, what benefit is there from doing exhaustive and time-consuming garbage collection just before exiting and after all productive work is done? When a process exits, ALL the memory is released back to the kernel - interpreter memory, program-space memory - all of it. Circular references or not. Isn't this last minute gc just a big waste of time for the cases (outside of persistent runtime environments) where any garbage would be collected?

Lots of reasons :-)

• Speed. Reference counting is usually slower than mark and sweep.
• Memory leaks are important even for stand alone scripts. Sure decent memory management might not be an issue with a script if it's processing eight files. What about eight thousand? What about eight million?
• Simplicity. You get to write simpler code if you have to deal with self-referential data structures. No more tedious mucking about with weak references or proxy objects to avoid memory leaks.