It turns out that putting use DBD::Oracle at the top of the script is usually unnecessary. The DBI module will load the appropriate DBD module itself based on your connect string, e.g. $dbh = DBI->connect("dbi:Oracle:$dbname", $user, $passwd);.

Although there's no harm in loading DBD::Oracle explicitly, you only really have to do it when you want to use Oracle's type constants: use DBD::Oracle qw(:ora_types);