Q1: AFAIK there are 2 ways of externally modifying @INC
1. PERL5LIB environment-variable
2. -I<directory-to-library>
Q2: only the directories in @INC, and not subdirectories are searched for modules. Of course for the module Data::Dumper all directories in @INC would be searched for a subdirectory called 'Data', with a file called Dumper.pm.