How do things work in a perfect world? The book of Genesis tells us this much: every living thing lives in harmony, food is plentiful, there is no such thing as pain, and nobody knows the difference between good and evil.
That's the loophole the serpent uses to convince Eve to eat the fruit from the Tree of Knowledge of Good and Evil. "God said not to because I'll die," she protests. "You won't die," the serpent says. "You'll just be wiser, like God, and see things the way he does." So Eve eats the fruit because she can't conceive of anything that isn't perfect, and if God is wise, then wisdom is perfect too. As for Adam, the Bible never really attributes any motive to his deed. He just seems to take the fruit from Eve without question.