Thanks for both replies! I see now; somehow I was fixated on the fact that the sorting sub or block is supposed to use the values $a, $b, but for a "random" sort, the whole point is that the actual values of $a, $b in each case are supposed to have no (deterministic) effect on the resulting order.
(By the way, to get a random order, my method would likely be to use rand (in some way) to select one of the 20 numbers, then use it again to select one of the remaining 19, etc. That may be essentially what someone has described in one of the replies already.)
chas