The generational GC means that a garbage collection run doesn't need to inspect everything every time it runs. That is, it is possible to ask it to only inspect some subset of the total allocations during a given run. Useful when you know that you have just released a bunch of stuff local to some level of stack framing, and you can free that with incurring the cost of having everything prior to the current level also inspected.

It has nothing to do with releasing memory back to the OS.

In general, Perl takes the view that if you asked for a lump of memory once, then freed it, then you are quite likely to ask for it again, so why bother giving it back to the OS. If you don't make further use of it, and other tasks require more memory than is physically available, then the unused portions of your task space in Perl will likely get swapped to disk. If you never make further use of them, then very little overhead is incurred by it's presence.

What is your concern with releasing memory back to the OS?