Can't delete row with DBI.pm

[Tessio F.]

Hello,
I have an database with a two column primary key:

create database contacts(
username char(20) not null,
contact char(20) not null,
primary key (username, contact)
);

I'm trying to delete a row with the command:
(connect to db..)
my $sth = $dbh->prepare("DELETE FROM contacts
                         WHERE username=? AND contact=?");

$sth->execute($name, $contact);

But it is not working. To make thing worse, I'm not receiving  any error
message either. What is wrong with my code?
Thanks for your time!