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;
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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.

-Matt