Place red wine, prunes, black pepper, and cinnamon in a saucepan. Bring to a boil, stirring often. Reduce heat to low, cover, and simmer until prunes are tender, about 5 minutes. Remove from heat; drain prunes, reserving wine, and allow to cool.
Cut a deep pocket almost all the way through the tenderloin. Rub salt over the tenderloin. Fill the pocket with the cooked prunes; secure opening with toothpicks.
Heat olive oil in a large skillet over medium-high heat. Cook fillet in the hot oil until browned, about 2 minutes per side. Pour in reserved wine. Cover until simmer until tenderloin is tender, about 1 hour. Discard toothpicks. Cut tenderloin diagonally into thin slices.
