So that's what runtime polymorphism might be good for. In this particular case, I like it better than the classic subclassing approach.

So your point is that you don't like polymorphism as provided by standard OO (at least in this case), and therefore decide to re-implement it manually?

Somehow that fails to convince me, and painfully reminds of attempts seen here at perlmonks to re-implement pack or unpack, CGI::param(), various core modules and other stuff.

I like the Haskell's approach of pattern matching against signatures:

divides x 2 = $(special cased implementation)
divides x y = $(general-cased implementation)
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It seems that to me that replacing object methods at runtime is a rather limited solution to this problem, because it works only for methods (not for subs, although they'd profit from the same mechanism), and more importantly, it only works for special-casing on the object. As soon as you want to special-case on the parameter (say, you know it's a prime, and thus always return false).

I don't know what's the best perlish approach is, but re-implementing an existing technique and thereby retaining the limitations doesn't exactly seem right.