$i   = 0;
         @arr = (1,2,3,4);
         $mid = $#arr/2;

         while ($i <= mid) {
           $tmp        = $arr[$i];
           $rear       = $#arr-$i;
           $arr[$i]    = $arr[$rear];
           $arr[$rear] = $tmp;
           ++$i;
         }
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Originally posted as a Categorized Answer.