Does the choice of the base affect the perimeter of a triangle?

No.

The perimeter is the distance all the way around the triangle.

So it's the sum of the lengths of the three sides.

The sum of three numbers doesn't depend on what order you

add them ... I think that's the 'commutative' property of addition.

So it doesn't matter which side you start with, or even what order

the sides are arranged in. The perimeter is always the same.