Perl 5.6+ already have a 'method' attribute for subroutines (sub f : method { }), but it currently doesn't do much afaik. You're feature could easily be turned into an attribute itself, which wraps the the method with code to check that $_[0] is from the proper class.

On the other hand with good documentation for your code and a clear interface you shouldn't really need to check that a method is being called as a method anyways, because your interface should be consistent enough to make it obvious