DBI Mysql insert problem

The start field in an MYsql table is defined as follows:
start TIMESTAMP

I am using DBi::Mysql and am trying to update a column as follows:

$sth = $dbh->prepare_cached('update tests set start=? where db_id=?');
$sth->execute('NULL', 1);

The last statement returns 1 (one row updated), but I cannot see it it the database.

What I noticed also, the server connection goes away after a number of like updates issued, however the server logs show nothing about update statements.
Something fishy is going on on the client side.

Using $dbh->do('update tests set start=NULL where db_id=1') works,
doing the same from a command line works as well.

I am using perl v5.8.8 built for i686-linux, DBI version 1.58, DBD::mysql version 4.005, MySQL Version: MySQL 5.0.45-community-log.

Any clues ?