the perimeter of a circle is its total circumference, but in this case a semi-circle will measure half the circumference plus its diameter.
If the semicircle measures 8cm then the total circle is 16 cm, and we can calculate its diameter or radius from there. The perimeter is defined as:
p = pi*diameter = 16
diameter = 16/pi = 16/3.14
diameter = 5.1 cm
Therefore the semi-circle perimeter is:
perimeter = semi-circle + diameter = 8 + 5.1 = 13.1 cm