PostgreSQL arrays and DBD

Hello.

I create a table:

CREATE TABLE groups (
group_id serial PRIMARY KEY,
name varchar(64) UNIQUE NOT NULL,
guests integer[] DEFAULT '{}'
)

I add a new record to the table:

INSERT INTO groups (name) VALUES ('My friends');

Now the table contains 1 record:

| group_id | name | guests
+----------+------------+--------
| 1 | My friends | {}

I read the new record from the table using DBI:

my $sth = $dbh->prepare(qq/SELECT * FROM groups/);
$sth->execute();
my (@guests, $group);
push(@guests, $group) while $group = $sth->fetchrow_hashref(); # Line 4
print $guests[0]->{guests}->[0]; # Why ({group_id=>1, name=>'My friends', *
guests=>[0]*}) ?

Output of the script:

Argument "" isn't numeric in null operation at ./guestmanager.pl line 4
*0*

DBD should return a reference to an empty array. But DBD returned the
reference to the array containing 1 element (0). How can I have a different
result:

({group_id=>1, name=>'My friends', *guests=>[]*})

PS
Version of DBD::Pg is 2.9.0 .