my $sql = sprintf "UPDATE table SET id = %d - id WHERE id IN ( %d, %d )", 
             ($val1+$val2), $val1, $val2;

mysql> select * from test;
+------+------+
| id   | num  |
+------+------+
|    1 |  100 |
|    2 |  200 |
+------+------+
2 rows in set (0.00 sec)

mysql> UPDATE test SET id = 3 - id WHERE id IN (1,2);
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2  Changed: 2  Warnings: 0

mysql> select * from test;
+------+------+
| id   | num  |
+------+------+
|    2 |  100 |
|    1 |  200 |
+------+------+
2 rows in set (0.00 sec)

mysql>