I believe this is so that builtins aren't looked up every time they are called, which is almost certainly for speed reasons. Where the fact that this 'builtin overriding behaviour needs to be implemented at compile-time' is documented, I'm not sure. It might be documented in the 3rd Camel, but I can't seem to find hide or hair of it in the standard docs. A patch to the Overriding Built-in Functions section of perlsub is probably needed.