#!/usr/bin/perl
use lib "/path/to/extra/lib";
use CPAN;  shell;

##</code><code>##

(for csh-style):
PERL5LIB=~/lib
export PERL5LIB
perl -MCPAN -eshell

(for bourne-shell-style):
PERL5LIB=~/lib perl -MCPAN -eshell