So I'm still not entirely convinced one way or the other!

Sorry guys (you and your colleague), you are arguing about silly things. Of course, your colleague is right. Perl will never optimize away arrays with the same content. It is just silly for perl to run through all arrays trying to see if they have the same content. But your colleague is completely wrong using array references as keys.

Have you actually tried to run the code you gave above? If you run it with "use strict;" then you'll get error "Can't use string ("ARRAY(0xc80de8)") as an ARRAY ref while "strict refs" in use". This is because array that you have given when you created hash is already gone, so perl would try to create new NAMED array and the name for that array would be ARRAY(0xc80de8) or whatever is the unique string that has identified original array.