A while back, I did a simple test of a C program mallocing a 4 MB chunk of memory, then freeing it. The memory was returned to the OS on Windows and Linux -- but not on every other of the 12 different (mostly Unix-derived) platforms I ran the test on.

Whether the memory is returned to the OS or not depends on the implementation of the user-level malloc function. Traditional Unix implementations of malloc simply call sbrk(2) to increase the address space of the process. Since this call is expensive, when you call free, they typically do not return the memory to the OS, but keep it to reuse next time you call malloc.

glibc 2.x based Linux systems use ptmalloc, which is based on Doug Lea's malloc. This version of malloc is able to return memory to the OS because it uses mmap(2), rather than sbrk, when the chunk of memory requested is larger than some threshold value (typically 128K).

MSVC CRT malloc similarly uses a "small block" and "large block" heap heuristic -- but uses Win32 heaps, rather than sbrk/mmap, under the covers.

So, if you were desperate to have memory returned to the OS, you could go to the bother of building a custom perl which used a different implementation of malloc.