Heat a nonstick skillet over medium-high heat. Add pear to pan, and sprinkle with sugar. Cook 2 minutes on each side or until golden.
Brush cut sides of bread with vinegar. Arrange pear slices, arugula, cheese, and prosciutto evenly over bottom half of bread; cover with top half of bread.
Heat a large nonstick skillet over medium heat. Add stuffed loaf to pan. Place a cast-iron or heavy skillet on top of stuffed loaf; press gently to flatten. Cook 4 minutes on each side or until bread is toasted (leave cast-iron skillet on stuffed loaf while it cooks). Cut into quarters.