During the season's bloody, heartbreaking opening episode, Carl was pulled away from the group by Negan, who told Rick that he would have to cut off his son's arm, or he (Negan) would kill Carl. After proving that Rick's spirit was sufficiently broken that he would do it to save Carl's life, Negan stayed Rick's hand.
"I had no idea it wasn't going to happen," Riggs said. "When I was reading the script, I was like 'are they seriously going to cut my arm off?' I was scared to death."
Of course, now he can laugh about it a bit, but Lincoln admitted that it was a harrowing scene to shoot, and that the first episode was so dark and such a "painful thing to go through," that crew members had to walk away during production.
While the series is wildly unpredictable, and nearly any character can die at any time, the show has shied away from amputations on the main cast for the most part. Famously, Rick loses a hand in the comic books, and both producers and special effects team members have downplayed the likelihood of that happenign on the show, sometimes offering the explanation that there are things actor Andrew Lincoln would have to do on-camera that would be difficult and awkward one-handed, whereas the Rick Grimes of the comics doesn't have such concerns.
Of course, eventually Riggs's character did lose an eye, as he did in the comics, so it's not as though Carl has been getting off particularly light this season.