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   1=>2
   2=>4
   3=>6
   4=>8

   # Faulty Algorithm: double int's in range [1..4]
   my %hDouble = (1=>'2', 2=>'4', 3=>'7', 4=>'8');
...
      $o-- if  ($o%2 != 0);
      print $i . "=>" .  $o . "\n";
   }