# table specs
CREATE TABLE tasks (
      member_id MEDIUMINT UNSIGNED NOT NULL PRIMARY KEY,
      quota MEDIUMINT NOT NULL,
      logged TIMESTAMP NULL,
);

# perl code to decrease quota by 1

my $member_id = get_memberid();

my $sql = qq{ UPDATE tasks
   SET quota=quota-1
   WHERE member_id=?
};

# Assuming a connection via $dbh exists
$dbh->do($sql, undef, $member_id);