My best rule of thumb here is a very vague one: is there a thing?

it's not a very sensible rule, i know, but my experience has been that it's not worth working in an OO way unless some entity in your program is asking you to do it.

or to put it another way: if your central data structure is a hash, then it's quite likely that oo will help. if it's a hash of hashes, then read perltoot right away. otherwise, perhaps not.

Damian Conway's rules are much better than mine, as you would expect.