One of the most useful offhand comments that clarified hashes when I was learning Perl is the following gem from the Camel.

A hash lookup is linguistically "of". So for instance you would write:

$wife{Adam} = "Eve";
[download]

That reads, "The wife of Adam is Eve."

So when you sit down to do a problem, describe it to yourself. Whenever you start saying phrases like, "of", "lookup", "check if we have seen this before", and so on, that is generally an excellent sign that a hash fits, and the above tip is frequently a good guide to naming the hash as well!