In my experience object IDs almost always use the stringified reference as a hash key (the usual way for inside-out objects.) Or
Scalar::Util::refaddr is used (as in
Class::Std and PBP.)
refaddr returns the numified reference, but Scalar::Util already turns warnings off when munging that. (And SU does it in a rather bizarre way with a regex and hex instead of using direct numification for reasons that remain mysterious to me.)
I don't think there are many common uses of numified references outside those two contexts.
Update: Actually it looks like the issue with refaddr is fixed in recent versions. It uses int now.