Look at Attribute::Protected and Attribute::Property. More specifically, look at their source to see what they're doing. Essentially, both are inserting some extra code that runs (fairly transparently) in advance of the code block you provided with the subroutine definition.

There are legitimate issues around when attributes get run (i.e. during the CHECK phase by default) and whether this is mod_perl safe, but this can be coded around as some others have mentioned. Personally, I think they're an interesting concept that hasn't matured properly yet. Essentially, it's an attempt at specifying behavior as a bunch of building blocks -- another way of templating functionality and reducing redundant code.