After upgrading to Snow Leopard, I had to redo the installation of DBD::Oracle. This time I used DBD::Oracle 1.23. The good news is that the majority of my previous instructions no longer appear to be necessary: a normal build of DBD::Oracle looks ugly, but completes.
However, I did discover that if you are running a 64-bit perl, which you probably are, you need to get the 64-bit Oracle instantclient release. If you build DBD::Oracle and then cannot load the module because of missing symbols (e.g., _OCIAttrGet), then what is happening is that you linked against 32-bit libraries which cannot load from the 64-bit perl instance.