"Considered Harmful" Essays Considered Harmful
Seriously, there's something I find questionable. You show how chained mutators can be abused and dangerous, and the solution is "please don't" write the mutators to support this usage (unless I misunderstood and the "please don't" only applied to doing the chaining, rather that writing the mutator that way). Going back to the famous "goto considered harmful essay", that would be like saying that, since goto can be abused, languages shouldn't have a goto. While this is a nice purist position to take, decades after the essay many real-life, pragmatic language designers include goto anyway and leave the responsibility of using it wisely (if needed) to the user. I choose to do the same: I allow the mutators I write to be chained, even if in practice I only chain them in limited cases where they are really clear (in my opionion).
Of course, if you really want to return an error code from your mutators, please go ahead and be consistent. I think returning the value that was set is useless, as others have said in this thread. But if the choice is between an undefined return value and returning $self, why not return $self and let the user do whatever he or she wants with it?