I don't have a tnsnames.ora file on my windows box, so the connect syntax I use is:

DBI->connect("dbi:Oracle:$host:$port/$service_name", $user, $password,
+ {RaiseError => 1});
[download]

ActiveState comes with DBI and DBD::Oracle these days, so I'll assume it's not a library installation problem (unless you have an old build of ActiveState perl). Also, you don't mention the error you're getting...so what is it?