Read further down in the Guttman/Rosler
paper to where they discuss the "packed-default" sort:
map substr($_, 4) =>
map pack('C4' =>
. $_ => @in;
The idea is to pack the entire list once before sorting
it then get each address back out with a substr afterwards.
Using this technique, you only make N packs and N substrs,
instead of N*log(N) packs. It also has the advantage of
using sort without calling a subroutine for each comparison
which is a huge speedup.