Yeah this hint is way too vague to yield the answer except by guessing -- "infinite perimeter" is just not the thing that characterizes fractals, and my mind went to a million other places first. Maybe a hint about self-similarity at every scale, or something along those lines, would be better.
Accept Brackets as well. Also, miles per hour & metres per second question would have speed(or even velocity) as more apt answer than rate. Rate is too broad of a term to just be defined by distance per time.