Cause for me was an old copy of /usr/lib/perl5/auto/Cwd, I renamed this directory and perl picked up the copy installed by the perl-base package and continued with the upgrade.

Currently have no idea how this old copy of Cwd came to be here, dpkg says it doesn't know, can only think maybe a makefile mistake when installing stuff using cpan or some such. If folk can prove it came from a Debian package a bug report would be appropriate.