CPAN.pm refuses to upgrade PDL on most of my systems:
Testing/Installation stopped: allow_installing_module_downgrades: ETJ/PDL-2.097.tar.gz (called for E/ET/ETJ/PDL-2.097.tar.gz) contains downgrading module(s) (e.g. 'Test/Deep/PDL.pm' would downgrade installed '0.22' to 'undef')