On the advice of juster on the Arch Linux forum, I deleted all files under /usr/bin/site_perl, /usr/lib/perl5/site_perl and /usr/share/perl5/site_perl to remove the duplicates installed by the cpan shell.

Then I reinstalled CPANPLUS::Dist:Arch and used cpanp to reinstall the modules I needed (i.e. Math::Combinatorics).

Now it's all working again. Thanks for everyone's help.

cheers, Chris