It isn't loading your Config.pm, so how would perl know to warn you?

If on the other hand, you knew enough to use lib or -I to get perl to load your Config, you should know enough not to use already used names

You could easily write something to warn you, using Module::Find, Module::CoreList, Module::Mapper, Module::Finder, Module::Locate, Devel::Modlist, Module::Which, Module::Which::List, so that

use warnings::modulecollision;
[download]

would warn you, but I don't know of any programming language that would try to do that by default, but I don't know of any languages that try to do this at all :)

People seem to catch on pretty quick, its like house numbers, you can have many with the same number, as long as the street names are different :)