What I meant by "reasonably clear" is that, though it could have been stated better, no other interpretation that makes sense would lead to a different answer. (Unless you think sheep can hop over the fence, which is a different issue.)
Yes, the arrows are meant to remind you of a line, which extends forever; in a real-world context, that means it extends far enough that you don't need to pay attention to the ends. (Physics teachers don't mind talking about infinite wires.) If you believed it was meant literally, would that change your answer to the problem?
If I had written the problem, I might have made the fence 100 meters long, but that might make some students worry that they had to use that number somewhere. I imagine the author was doing his best to keep students from worrying about the rope wrapping around the ends (which would make it a much more interesting problem, but not what was intended).