What type of fence? Electric poultry netting?
If netting, you may want to keep it as is, since 100 linear feet is only 25 feet square.......20' x 50', etc. When fully laid out, 100' makes a nice sized pen.
But if you must shorten it, it can be overlapped.......or if you REALLY must shorten it, cut it off about 5 inches or so past a post, then wrap the excess around the post and tie it back upon itself. Save the rest to splice it back together someday.
If it is wire or poly tape, it can be cut and spliced like rope. Just tie the ends together.