After anything else, thank you for your interest, abigail.
Here is some more work on the same issue:

• I'm using a legacy CGI environment, I don't have any ideas about how to maintain connection caching here.
• I'm using MySQL (legacy, too), so I can't use stored procedures: there is no support for them here.
• I'm currently using indexes and I'm sure that data is stored as closer to the third normal form as possible.
• I'm very limited on the database server available memory. MySQL is doing the best it can on keeping data on memory.
• My tables are legated, too, and I can't transform things that much (changing data structs don't worth the effort).

The perl-related part is: without changing things at the database (this could break other applications that use the same data), I need to develop a strategy to control changes, and built a UPDATE SQL query to apply changes to the database, minimizing the data writen. If you know any modules that could be useful, or any strategy that I can implement, please let me know.

Thank you very much for your attention.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
monsieur_champs