They are unnecessary in this instance (and I'm lazy enough to leave out the quotes most of the time also), but I don't see how they'll cause trouble later. Actually, they'll save trouble if he decides to change the keys to something that is a perl keyword, or if perl changes and suddenly 'this', 'that' or 'myhash' becomes a perl keyword.

Update:Hmm, tye right. runrig wrong (see below). Key words and function names don't cause problems, but how 'bout this:
If you decide to add non-word characters later, it can cause trouble unless its quoted so it may save you from remembering to quote it. Or if you later decide to use a hash slice, you'll have to quote the arguments anyway. Or some people just think it looks better, so who am I to argue :)