This particular module should be useful without other (non-core) dependancies, and should run on Perl 5.6. There is another place Scalar::Util would come in handy (to distinguish a floating-point literal from a string literal), but I'm trying to do without.

I thought about using isa rather than ref, but figured the latter is faster and I know exactly what the possibilities are. That is, I won't see a derived class coming back in.