Because $a and $b are assigned items from the list you're sorting, not indexen. You either want to use $a->function and $b->function, or sort a list of indexen (sort { $unsorted[$a]->function ... } 0..$#unsorted).

Also if $unsorted[$idx]->function is expensive (or possibly changes during the course of the sort) you might want to do a schwartzian transform first and just make one set of calls.

Update: Oop, you know if you don't know what sort passes to the sort block you probably don't know what an ST is . . .

@sorted = map { $_->[0] } sort { $a->[1] <=> $b->[1] } map { [ $_, $_-
+>function ] } @unsorted;
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(Of course were this Ruby that'd just be unsorted.sort_by { |x| x.function }; how easily one can get spoiled . . . :)