In the book of Genesis, God tells the serpent "I will put enmity between you and the woman, and between your offspring and hers; he will crush your head, and you will strike his heel." The serpent is commonly thought to be Satan. In Perelandra, the Un-Man (a representative of Satan) bites Ransom's heel, and Ransom crushes the Un-Man's head with a rock. Very clever, Mr. Lewis.
In the book of Revelation, Jesus promises that the one who overcomes will inherit the morning star, which is another name for Venus (Perelandra in The Space Trilogy.) In Second Corinthians, Paul mentions someone who was caught up into the third heaven, which could also correspond to Perelandra.