NULL values returning as empty string

I am unable to figure out why but NULL values which from what I read should be undefined are being defined. See the example below for the column thold_hi which shows as NULL in the database but the reference $row->{'thold_hi'} being defined.

I am running mysql 5.0.22 and DBI 1.58.

Example:

my $sql = "SELECT t.lastread,t.thold_hi,t.thold_low,thold_alert,bl_alert FROM host h LEFT JOIN data_local dl ON h.id=dl.host_id LEFT JOIN data_template_rrd dr ON dr.local_data_id=dl.id LEFT JOIN thold_data t ON t.data_id=dr.id WHERE h.description='something' AND dr.data_source_name='hdd_free'";

my $sth = $dbh->prepare($sql);
$sth->execute();
my $row = $sth->fetchrow_hasref();

if (!defined($row->{'thold_hi'})) {

        print "Not Defined\n";
} else {

        print "Defined\n";
}

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mysql> SELECT t.lastread,t.thold_hi,t.thold_low,thold_alert,bl_alert FROM host
h LEFT JOIN data_local dl ON h.id=dl.host_id LEFT JOIN data_template_rrd dr ON
dr.local_data_id=dl.id LEFT JOIN thold_data t ON t.data_id=dr.id WHERE h.description='something' AND dr.data_source_name='hdd_free';
+----------+----------+-----------+-------------+----------+
| lastread | thold_hi | thold_low | thold_alert | bl_alert |
+----------+----------+-----------+-------------+----------+
| 24997856 | NULL | 25000000 | 1 | 0 |
+----------+----------+-----------+-------------+----------+
1 row in set (0.00 sec)

Telesphere Networks, Inc

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