By definition you can not have more than one quintessence of
something(1). I do agree that inheritance is the
fundamental element or the typical characteristic of OO.

(1) Apologies to the Humpty Dumpty-ists among us.

That does not mean that every class should be inheritable.
A mechanism to prevent the inheritance of classes is a
good thing. That inheritance was not anticipated during
the design of a class, or a the usage of a class being
deprecated are two reasons off the cuff.

I don't know of such a mechanism for Perl. In Perl
this might be expensive.

To the initial query:
Start another package to avoid disturbing the your existing
clients.

I appreciate the return_with_object idea, but
see it as necessary in certain social situations. Coding
so as to protect oblivious coders seems like a silent
endorsement of oblivion. I realize this is not necessarially
so, nor always a bad thing; but to write to a level
below your abilities cannot be good in itself.