It's not null. Null is a value (or at least the specific absence of one). It's uninitialized - there's never been a value assigned, and although Perl will try to do the right thing and assume 0 in numeric context or the empty string in string context, it really is just what it says: uninitialized.
Update: Just remembered, some DBs use Null the way you did, I think.--Bob Niederman, http://bob-n.com
All code given here is UNTESTED unless otherwise stated.