I don't see the point of this. AIUI, on exit, perl's garbage collector will free that memory anyway.

Because the OP asked how to do it.

And there's plenty of reason to want to learn how to perform cleanup.

As for the specific case, it's a question of debate whether you should free memory on exit. But it does have benefits. One major benefit is that allows us to use valgrind or similar to find meaningful leaks.

Finally, you're incorrect assuming that the termination of a Perl interpreter only occurs when the program is unloaded (i.e. when process exits, execs, etc). Each threads of a process has its own interpreter, for example. A process embedding Perl could load and unload the interpreter without exiting.

Is there any reason to believe that it would be different under FFI::Platypus?

There's no reason to believe it would be the same.

And the OP specifically quoted someone saying it was different (that it results in a NULL argument).

If there's a bug, fine. Point it out, fix it, whatever. But it has nothing to do with my answer. The question is about ensuring that cleanup is performed, and that's what I was addressing. The exact interface and implementation doesn't matter, so that's a separate topic you've already addressed and didn't need to bring up here.