How often are you planning on upgrading? If this is a one-time thing, don't get yourself buiried in the complexities of threading. If this is something that might happen once or twice a month, and if it takes longer than a single night to upgrade everything, then you can probably get a speed boost with parrellizaing. Alternatively, you can make one program that can take a portion of the list of hosts and run it multiple times, each run with a different section of the list.

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I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated