my $myClient = 97;
my $deliverymethod = 'USPS';

my $sql =<<"__SQL__";
SELECT
            ORDER_ID,
      ....
            CONVERSION_SERIAL
FROM  INTEGRATION.OL_ORDER
WHERE
            CLIENT_ID = $myClient
            AND BATCH_ID  =
                (SELECT * FROM (SELECT
                    BATCH_ID
                    FROM OL_ORDER
                    WHERE CLIENT_ID = $myClient
                    AND DELIVERY_METHOD ='$deliverymethod'
                    GROUP BY BATCH_ID
                    ORDER BY BATCH_ID DESC NULLS LAST)
                  WHERE ROWNUM <= 1)
__SQL__