Always a million ways to skin a cat, and here is yet another...

#initialize array
@array = (12,7,4);

$i=0;
foreach (@array) {
   #Throw in a hash to look up in a second
   $hash{$_} = $i;
   $i++;
}

#sort it, loop through, and look up from hash from above
foreach (@num_sorted_array = sort { $a <=> $b } @array) {
  print "$_ came from \$array\[$hash{$_}\]\n";
}
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Probably not the most efficient way (as I loop through array twice and throw into / lookup to a hash), but simple enough.

Regards,
nandeya