To make the left-hand side of the equation \( x^2 - 8x + c \) a perfect square trinomial, we need to complete the square.
The general approach for completing the square is to take the coefficient of \( x \) (which is -8 in this case), divide it by 2, and then square the result.
- Take the coefficient of \( x \): \(-8\)
- Divide it by 2: \(-8 / 2 = -4\)
- Square the result: \((-4)^2 = 16\)
So, the value of \( c \) that would make \( x^2 - 8x + c \) a perfect square trinomial is \( 16 \).
Thus, the answer is \( \boxed{16} \).