Those are two simple questions. The second is easy to answer: DBD::Oracle does NOT need -Dusethreads. But when you build perl to be used for Oracle, be sure to link in -lpthread. You might also need -lcl.

The first question really depends on what client library you are linking DBD::Oracle to. A 32bit client library will talk to a 64bit server (even on the same machine), but if the database is running as 64bit on the same machine, it is much easier to link to 64bit client libraries. The bitness of the perl binary has to match the bitness of the client library.

If your Oracle installation is installed with both 32it and 64bit client libraries, you are free to pick the one that matches your perl.