Interesting idea. Eiffel had it ten years ago (of course, no-one uses Eiffel, ha ha ha!).

The idea was to add the idea of deprecatedness into the language. Meyer achieved this by adding the obsolete keyword to the language. A method or class could be labelled obsolete, which meant that whenever new code was written that used it, the compiler would spit out a warning (but would not in any other way affect the semantics, you could continue to use it and it would work correctly).

Have suffered C macros that wrapped function declarations with warning pragmas at the time to emulate the concept poorly, I found the idea appealing. I still do, I guess.

As for manipulating the symbol table, it is not particularly difficult to intercept calls to subs and wrap them up in your own stuff. I must say it's one of the more exhilarating experiences one can do in Perl. First time I did it, I kept thinking, "ha! try doing that in C."

update: minor wordos tweaked.