I'm not familiar with specifics, but I believe that Oracle does provide tools for profiling the performance of the query engine, as well as analyzing specific queries. You might need DBA-level access to find and use them.

As for perl/oracle interaction, a few general rules of thumb are usually sufficient. Some have been cited in previous replies; another I would add, which could have major impact, is to use the oracle-native sqlload(er) tool to do bulk insertions into tables, rather than iterating over lots of DBI "insert into..." executions -- even when you use prepared/placeholder statements, bound variables, etc, in your DBI insert calls, this is still one or two orders of magnitude slower than sqlload.