Update: I've had a productive discussion on this subject on the dbi-users mailing list. If you're interested, please see this thread.

The conclusions were:

1. Oracle does close ref cursors automatically when the variable holding them goes out of scope;
2. I probably need a version of DBD::Oracle compiled to use the Oracle 8 API; and
3. my problem is probably to do with a reference being left open somewhere.

Hope this is helpful to someone.