It doesn't work. Well, it does work, but only if the XFORM routine returns a string of the same length for every item. Otherwise, the sorting could turn out wrong.

I've made a very contrived example, containing lots of null bytes, but actually, if you have a sufficiently large amount of array items, you can get the same effect on other bytes as well.

Let me demonstrate the effect by sorting a number of variable length strings as is, and with the packed index appended. As is shown, the sorted results are not in the same order at all.

use Data::Dumper;
$Data::Dumper::Useqq = 1;
my @data = map "\0" x $_, 0 .. 5;
print Dumper [ sort @data ],
  [ sort map pack("a*N", $data[$_], $_), 0 .. $#data ];
[download]

Result:

$VAR1 = [
          "",
          "\0",
          "\0\0",
          "\0\0\0",
          "\0\0\0\0",
          "\0\0\0\0\0"
        ];
$VAR2 = [
          "\0\0\0\0",
          "\0\0\0\0\0\0\0\0\5",
          "\0\0\0\0\0\0\0\4",
          "\0\0\0\0\0\0\3",
          "\0\0\0\0\0\2",
          "\0\0\0\0\1"
        ];
[download]