To add to this:
No, only once the process has exitted will the memory be available to to other apps.
This is correct, but needs to be qualified. Physical memory (actual RAM) that was taken by the variables (or better, memory pages) that are no longer used can be reclaimed: the pages containing the unused variables get swapped out to disk if some other process needs physical memory ('in core' memory). But the variables do stay in swap. They are only removed from there when the Perl process exits.