Well, I would think it is because in sub rotate, you are assingning the data in @_ (x,y,z) to the aliases (references $x,$y,$z) indexed @_[-1..($#_-1]. However, in rotate_not, you are assinging the data (array slice) from @_[-1..($#_-1] to the array @_ and not the references. So you are overwriting the references to $x,$y,$z and not updating them.

zzSPECTREz