reverse sort { my_cmp($a, $b) } @foo    # (1)
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and

sort { my_cmp($b, $a) } @foo            # (2)
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are not strictly equivalent: order is reversed for terms comparing as equal.

For instance:

@u = map { [$_, $i++] } (0,0,3,2,2);
$" = '-';
print( (map {$_->[1]} reverse sort { $a->[0] <=> $b->[0] } @u),
       "\n",
       (map {$_->[1]} sort { $b->[0] <=> $a->[0] } @u),
       "\n" );
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prints

24310
23401
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though I believe that most of the times, the correct solution is actually the second one or it just doesn't mind at all!