Custom `umask` will cause an incorrect permissions for some perl dirs, created during `cpan` usage.  As result only you will have access to the required perl libraries. It is importan if you wanna to share those libraries with other users. You need set `umask` to default value before execute `cpan`.

_The default mask value is S_IWGRP|S_IWOTH (022, write access for the owner only)._ (`man umask`)