Most people would die from being shot in the eye. I know when Denise got shot with a arrow in the eye, she died, so why didn't Carl? Don't get me wrong I'm happy he lived but why did he?
Getting shot in the eye is not what did or did not kill either Carl or Denise. The eye is not a vital organ as evidenced by the many people who live without functioning eyes. The reason Carl survived is because only his eye was damaged and he received adequate medical attention and was able to heal. Denise however, was shot through the head with an arrow. Her eye was damaged, but what killed her was that she was shot through the brain.
If you are interested, here is an article about people who have been shot in the head and survived. Its not impossible.