An assignment never copies, not for any type. It always rebinds the name z to point to the same thing that x binds (i.e. refers) to. If you’re thinking of something like z = x[1:3] and how that copies - it’s the x[1:3] part (the slice) that copies, not the assignment.
Also, your suggestion about : would change julia quite a lot - note that we’re not necessarily at that stage of development anymore.