First; are you sure your cpan command loads the config from /etc/perl/CPAN/Config.pm and not some other location?

Also; did you check that the contents of Config.pm matches what you think it should be?

Also; what is in your Config.pm and what is the error you're getting?

In any case, I'd be very surprised if CPAN.pm does any checks on Config.pm besides trying to use its content.