The Egyptians used a rope with 13 equally spaced knots in it, which gave them 12 equal sections of rope. These would be the units. One of the most recognized Pythagorean triples is 3, 4, 5. This comes from the Pythagorean theorem:
3²+4²=5²
9+16=25
With the sides of the triangle being 3, 4 and 5, the perimeter would be 3+4+5=12.