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.
$Data::Dumper::Useqq = 1;
my @data = map "\0" x $_, 0 .. 5;
print Dumper [ sort @data ],
[ sort map pack("a*N", $data[$_], $_), 0 .. $#data ];
$VAR1 = [
$VAR2 = [
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