If it's a REGULAR pentagon ... all sides the same length ... then yes,
because all 5 sides of any pentagon form the whole outline for the
perimeter.
But if it's not a regular pentagon, then the sides have different lengths.
You might get lucky, and pick one side that happens to be 1/5 of the
perimeter, but that would just be blind luck. For an irregular pentagon,
you can't say that the perimeter is five times the length of any one side.