Even when you undefine variables, the memory used is not returned to the operating system until the process terminates

I'm not a FreeBSD user (so this may or may not be true), but usually the OS can reclaim memory that's munmap'ed. Of course you'd also need a special version of perl that used mmap instead of malloc, and each variable would take a VM page of storage (4k on x86), but that's another story.