A couple threads suggest that it may be prudent to manually upgrade CPAN.pm itself before using it to install other modules.   That is, if your existing Perl installation is not the very latest, I think.

• Stopping CPAN from upgrading Perl
• Why does the CPAN module upgrade to perl 5.6?

Zaxo's sage advice should help with the manual upgrade, after which Cirollo's good counsel should help with using the CPAN shell (without having to worry about a bug in older versions of CPAN.pm).
    hope this helps,
    Don
    striving toward Perl Adept
    (it's pronounced "why-bick")