Perl doesn't know this because my.conf can be in a different place, you might not have a local mysql server, you might not want to connect to the local mysql, or you might have more than one local mysql deamon.

In other words; you need to supply the connection info yourself, because it's no use guessing.

update: the DBD::mysql manual states:

    $dsn = "DBI:mysql:database=$database;host=$hostname;port=$port";
    $dbh = DBI->connect($dsn, $user, $password);