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. Sprinkle generously with freshly cracked pepper. 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.