I can sort of confirm it, and sort of deny it.

It is true that at one point Perl would never return memory to the OS. However someone fairly prominent in p5p (I think that it was chip) told me a few years ago that he had seen a program that did return memory to the OS, had looked into how the trick was done, and taught Perl to do it with some operating systems. So while you cannot rely on memory being returned to the OS, on some operating systems under some circumstances, Perl might do so.