Optimized constant subs are stored as bare scalars in the stash (package hash), which formerly held only GVs (typeglobs).

But you cant create them manually - you cant assign a scalar to a stash element, and expect it to work like a constant-sub, even if you provide a prototype.

This is a feature; alternative is too much action-at-a-distance. The following test demonstrates - napier is not seen as a function at all, much less an optimized one.