I think about the best you can get is finding all the namespaces that have the sub (by that name). But if you look at the source for constant, you'll see that it exports a sub into your namespace without naming it in the constant namespace...

Since it does this without ever calling any magic subs, that is, it's just using a package glob-hash thingy (that people usually call a symbol table), there's nothing to overload, and likely no way to trap it. I mean... unless you tie the symbol table hash. I wonder if you can do that without breaking perl completely.