To appreciate the significance of the Buddha's perfect body, it is necessary first to consider the career of the Buddha as Prince Siddhartha, before he attained enlightenment, and then the body of the Buddha who has attained Buddhahood. The latter is discussed at great length in such canonical texts as The Sutta of the Excellent Signs (Theravada tradition) and The Sutras of Far-reaching Discrimination (Mahayana tradition).
Prince Siddhartha cannot be said to have had a perfect body, since he was only mortal. However, like everything else about his life, it was as good as mortality allows. The point here is that even a wealthy prince with a loving family who is handsome and healthy is subject to suffering. It is important that Siddhartha's body should be as beautiful and healthy as a body can be, in order for him to represent the peak of human experience.
The body of an enlightened Buddha, however, is truly perfect, since it is an expression of that being's enlightenment. The elegant proportions of the Buddha's body, and his smooth, glowing skin represent the way he has lived and the moderation and benevolence he has displayed. His teeth are even and white, because he has always spoken the truth. The page attached below, intended for advanced students of Buddhism, gives a list of the major physical signs of Buddhahood.