Rules are for users. :-)

Built in functions are exactly that, built in. Try calling them with an & and you will fail. If you have a regular function with the same name, the built-in takes precedence unless the regular one was imported into the current package. (See perlsub's section on overriding builtin functions for more.) Even then it only works if the function is in a list of things that can be overridden.

If it seems strange to you that something so important is not in the symbol table, neither are lexical variables...