I said:

a return code to signify what failed;

(emphasis added).

I fully realise that undef is a return code. It does not, however, carry information about what failed and why. The lack of defined return code is my signal to go off and look at my object/class and get the error.

That works for me (and as bryan notes above, is somewhat common in CPAN modules).

Your mileage might vary, as they say ;-)