The change is seen everywhere in the interpreter, and that's the very definition of global. In any case, it's definitely not lexical! The point is that changes to @INC could have very far reaching effects. The fact that the change is temporary doesn't change that.

I did originally imagine lib::lexical mainly as a syntax sugar for 'local @INC = (..., @INC)'.

That would be FAR clearer to the reader! (But just as dangerous and pointless.)