forgot to mention is that he's firstname.lastname@example.org
I am now email@example.com
I am new to this as well, but for others who don't have Mastadon-type experience: If you are on a different fediverse/activitypub network, I believe you can follow someone from perl.social by using their @firstname.lastname@example.org and/or https://perl.social/profile/username. Similarly, I was able to follow someone from a different server from my perl.social account using their profile URL from that server in your "Add new contact" field.
But because they cross-communicate, if you've already got a Mastadon or other fediverse/activitypub account, there is no reason to create another account at perl.social, and you can still follow folks that do. Since I hadn't chosen a server yet, I decided perl.social was a reasonble place to dip my toes into the fediverse.