The module you have problems with is really called "Config.pm" or is this just an example? Since there is a Config.pm in the .../perl/lib/ already I would expect a lot of errors if you prevent Perl from loading it (by loading your Config.pm first) as well as from not loading your module (if the core Config.pm was already loaded). Rename your module!

Next question is ... is the working directory what you think it is? I believe some versions of IIS used to set the working directory to the web root, not to the directory containing the script!

In either case your modules should not be in the web accessible directories, it's better to have them outside the cgi-bin.