Contrary to what I wrote here on 3rd december, there is no definitive way to figure out whether a built-in function is overridable, apart for trying to override it, or reading perl's source code. There's several kinds of magic at work in there you know. Now I begin to understand why there are so many quotes from Lord of the Rings in perl's sources.
(I've submitted a doc patch to perlsub that should make things more clear -- esp. in this area where there was really little docs.)