Thanks for your reply.
The error message is from CPAN.pm, one of the files on my own
system. I believe that this perl program is looking for the directory "authors" in my $HOME/.cpan. In fact, the correct pathname is $HOME/.cpan/sources/authors, so it should be trying to cwd to "sources/authors". But I could be wrong about what the code means. I guess I could check it out by making a symbolic link to get round this error. But I would prefer to configure cpan correctly so it doesn't happen.
David