It's not just that the key is computed, but that the key has an internal sort comparison which is way faster than a user code comparison. It was an example of

It's not just that the key is computed, but that the key has an internal sort comparison which is way faster than a user code comparison. It was an example of the fact that "Big-Oh notation is nice, but constants sometimes matter." Both sorts have the same O(n log n) number of operations performance. However, in Perl, using an internal comparison for integers was a significant constant faster than using a user specified comparison function--at least an order of magnitude and sometimes two. Consequently, the 2*O(n) to decorate and undecorate the list with integers to sort by got swamped by the 1/10*O(n log n) sort improvement. I believe that this overhead eventually got reduced. However, I remember using this a LOT in Perl 4 and early Perl 5 days.