From the command line,
perldoc Config
You shouldn't change that file.
I think the following will do the trick to install Authen::PAM:
wget http://search.cpan.org/CPAN/authors/id/N/NI/NIKIP/Authen-PAM-0.16.tar.gz
tar xvzf Authen-PAM-0.16.tar.gz
cd Authen-PAM-0.16
perl Makefile.PL CCFLAGS='-I.'
make test
make install