DELIMITER //
DROP PROCEDURE IF EXISTS DebugMe //
CREATE PROCEDURE DebugMe(OUT arg char(10))
BEGIN
    SET arg = "HELLO";
END //
DELIMITER ;
CALL DebugMe(@blah);
SELECT @blah;

Query OK, 0 rows affected (0.00 sec)

+-------+
| @blah |
+-------+
| HELLO |
+-------+
1 row in set (0.00 sec)