In fact, a Perl hash is an efficient implementation of what some languages (as well as Computer Science as a discipline) often refer to as an associative array There happens to be a good wikipedia article on that search term, and though it doesn't discuss Perl-specific implementations, it does a good job of explaining the principle.

Something else associative arrays are good for is representing sparse arrays (with the caveat that Perl hashes don't maintain any particular order).

But they excel at several things: First, very efficient lookups (In Big-O terms, O(1) lookups). Second, enforced uniqueness of keys (which comes in handy for categorizing, eliminating duplicates, checking for 'seen', etc.).