Most perl modules can be installed into an alternative locations, via the use of "use lib" or other similar modifications. Assuming that what you are trying to do is allow users to simply drop in the code and necessary supports, I would try to work with modules that have a GPL-like license or similar that allows free distribution of that module code. Thus, you can now provide your users with a tar-ball of not only your code, but the modules that you need. The extra modules would then sit within the user's directory on the virtual host.

You can easily check for the existence of a module in a pre-install script to decide if it's need, possibly by the use of automake/autoconf, or even via makefiles though you'll need to do a few tests to make sure that work, or you can let the user decide to use that support module if necessary via your web-config idea.

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain