Exit the cpan shell, move the existing .cpan directory to a backup:

mv /root/.cpan /root/.cpanbackup
[download]

Restart the cpan shell and try again. Also in Ubuntu package management you're looking for libnet-snmp-perl.

Update: As a side note you may consider installing your own perl elsewhere, and leaving the system perl to the OS. You can easily install perl elsewhere as per the installation instructions, or perlbrew.