The teams announced the deal Sunday.
Perreault has played in 159 games with the Capitals, scoring 70 points with a plus-17 rating. He has won 51.2 per cent of his faceoffs.
Perreault began his career in Washington playing for Ducks coach Bruce Boudreau, who also coached Perreault in the AHL.
Mitchell is a left wing who spent last season with the Ducks' AHL affiliate in Norfolk.