my %lookup = (
   FAULT_NO => 'FAULT_NO',
   ISSUE    => 'ISSUE',
   RESPONSE => 'RESPONSE',
);
# task of automating the creation of the hash
# is left as an excerise to the reader ;)

##</code><code>##

$order_by       = $lookup{ param('order_by') };
$order_by_check = $lookup{ param('order_by_check') };

if ($order_by) {
   ...
}
...