To solve the quadratic equation \( x^2 - 16x + 64 = 0 \) using the perfect square trinomial pattern, we can recognize that this equation can be expressed in the form \( (x - a)^2 = 0 \).
We can rewrite the equation:
\[ x^2 - 16x + 64 = (x - 8)^2 = 0 \]
Setting the expression equal to zero:
\[ (x - 8)^2 = 0 \]
Taking the square root of both sides gives:
\[ x - 8 = 0 \]
Therefore,
\[ x = 8 \]
So, the solution to the quadratic equation \( x^2 - 16x + 64 = 0 \) is:
\[ \boxed{8} \]
The correct answer is D: \( x = 8 \).