Read further down in the Guttman/Rosler paper to where they discuss the "packed-default" sort:
@out = map substr($_, 4) => sort map pack('C4' => /(\d+)\.(\d+)\.(\d+)\.(\d+)/) . $_ => @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.