This isn't a Perl-specific answer but have you tried to optimize your queries? For instance, have you:
Looked at your query code and removed any functions in the where clause (they have a tendency to cause full table scans)
Run EXPLAIN PLAN to find out how, exactly, your queries, are running
Considered CONSOLIDATING these six queries into one or more stored procedures. The fact that you are running these one after another jointly seems to indicate that they MAY work better if you consolidated them into a single stored procedure.
Before you look into multithreading, I strongly suggest you take a very close look at the load you are putting on the Oracle server first.
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