we could have a special syntax for keys and values of a hash ( like e.g. %<hash ).

It couldn't be a variable since it's a list of values. It wouldn't make sense to give it the syntax of a variable since you couldn't do anything to it that you could do to a variable.

Unless you mean keys in scalar context. That's an lvalue function that returns a magical variable like substr. As such, that could be made into a variable (e.g. keys(%h) = 123; could be made into %#h = 123;).