it's not a perfect square trinomial because it doesn't factor into one.
The square root of c-term 64 = 8
then the b-term coefficient would need to be -16 not -8.
x^2 - 16x + 64 = (x - 8)^2
you can also see this by calculating vertex. perfect squares have one solution which means the vertex of a perfect square trinomial is on the x-axis.