I would definitely love to have a native given expression today. It does not mean that I believe it's likely, but certainly it's highly desirable.
The real power is not in given/when but in the smart match operator, ~~. I see no reason why it couldn't be implemented in current Perl, as infix ~~ doesn't clash with any existing operator. Obviously, the negated version (!~) can't exist, but not/! fixes that easily, and we could use even opt to use !~~ for the time being.
All we need is a volunteer to patch perl. :)