Carl is now in an interesting position. He's received a fatal zombie bite, and he's the only one who knows it. Another shot shows him writing some things down. In a post-mortem interview with The Hollywood Reporter, which was published just after the fate of his character was revealed, actor Chandler Riggs previewed the end of his character's story. "Episode 809 is really Carl trying to teach Rick as much as he can about what he's learned and trying to convince Rick to not kill every single Savior because there's still good people out there . . . there's some humanitarian aspects that Carl is going to try and teach Rick in his final moments."
Based off of this information, it seems Carl may be spending part of his final day on Earth writing letters. It's true that he may be teaching Rick some of these lessons verbally, but it's all the more impactful for Carl to give parting words in writing. Perhaps Carl is writing other letters, too: letters to Michonne, to Daryl, even to his little sister, Judith.