Alright, lets get started.
Please refer the diagram I have attached.
There is a triangle ABC having co-ordinates (1,3), (5,2) and (4,-1)
When we make a reflection of this triangle names A'B'C', the co-ordinates are changed (-1,3), (-5,2) and (-4,-1)
But the length of each side remains same.
So, if the length of each side will remain same after its reflected, so perimeter will also be same.
So,after reflection, the perimeter of a figure is not changed. : Answer
Hope it will help :)