"What I really want is a way to make $hash{bare_word} a compile-time error in a lexical scope..."

Terminology is a bitch. While it's true that interpreting a bareword as string, which occurs at complile time, is not called stringification, I think it is clear that OP wants $hash{blahblah} treated differently from $hash{'blahblah'} … only the former should be a syntax terror, something like use stricter; .-)