I can only speculate, like in my last update, that as soon as a sub has "weird" flags, the symbol table acts as fallback.

But I couldn't find any informations in the docs, we need to wait for someone knowing the internals.

Obviously is deleting a symbol, like already demonstrated, an efficient way to sabotage the whole redefine mechanism.

Update

Well, I should have added that undef &g is changing the sub considerably. Use Devel::Peek with a coderef to see by yourself.