So how do I determine if a value of a hash key is a string or a ref to another hash?

ref

is this the best way to represent this kind of stuff.

Looks pretty good. Objects are an alternative, but not necessarily better.

The aim of this code will be to compare a stored version of this data with the current version so we get notified of any changes made to the network, E.g. "workstation1 has been unplugged from hpswitch1."

A 2D array for the blade and port might make things slightly easier.