You're not really having a Perl problem, but I'll respond anyhow. Here's a MySQL session that highlights what you are seeing:

mysql> create table foo (i int);
Query OK, 0 rows affected (0.14 sec)

mysql> insert into foo values (1), (2), (3);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from foo where i = 42;
Empty set (0.04 sec)

mysql> select min(i) from foo where i = 42;
+--------+
| min(i) |
+--------+
|   NULL |
+--------+
1 row in set (0.04 sec)

mysql> select min(i) from foo where 1 = 0;
+--------+
| min(i) |
+--------+
|   NULL |
+--------+
1 row in set (0.00 sec)