why does perry start eating again?

Actually he is force fed.

By the beginning of June, Perry has stopped eating, and spends the summer in the prison hospital, connected to a feeding tube yet still refusing sustenance, determined to take his own life before it can be taken from him. One afternoon, however, he receives a card from his father, addressed to the warden, inquiring about his son; the card inspires a frenzy of “love and hate,” and Perry resolves to end his fast and return to the world of the living.