This blog post addresses checksum and signature verification vulnerabilities affecting CPAN, the cpan client, and the cpanm client, which were published in a security advisory on 23rd November 2021. If you're not aware of this topic, you might like to start by reading the advisory. This post gives a high-level description of the issues, what has been done to address them, what is still left to do, and what you should do. If you have any questions on this, you can add comments here, or email the PAUSE admins (modules at perl dot org).
Before we dig into the details, we'll first give an overview of how the relevant parts of the CPAN ecosystem work.
If you're not interested in the details, skip to the section "What do you need to do?"
TL;DR: make sure your CPAN client uses https and a trusted mirror – such as cpan.org