This is really something that falls within the responsability of the OS rather than Perl. When a process exit()s, the OS is responsible for closing all open descriptors, reclaim memory, flush caches, etc.

You should be concerned with leaving your application in a consistent state (ie, calling ->rollback() on pending transactions, etc). This, as you might see, lies on a higher level of abstraction than closing a file of freeing a block of memory.