Thank you for precious answers, the server-computer analogy made it all more clear :o) Still not perfectly clear, but a lot more.

1. Perl
I am thinking of using linked clones for the OS (Windows XP) and softwares, and maybe Perl's installation (i'm still not clear about this, i don't know why).

2. CPAN modules
For the modules, I read this article, which seems to answer my question.
If i understood well (all this is quite new to me), by using the scripts presented there, i would

- Download and install CPAN modules normally on my host computer

- Script "Listing 4" would read the list of my CPAN modules and download them to my shared folder

- Script "Listing 5" would build all of these modules to make them "ready-to-install"

- Script "Listing 6" would install them on a computer (repeat this operation for each computer).

This method is quite old (2004), is there any better way now? perlbrew?? I really don't know much about it, is it worth looking into, for my needs ?

3. Source code
Is it possible to have my source code only in one place ?
Could i then run scripts with WWW::Mechanize::Firefox from each of the computers?
Otherwize i guess revision control will be my solution.

PS : As i run all my guests on the same host, i think i can use VirtualBox's shared folder for the CPAN installation trick... and for my source code ??