What is the mechanism for perkin reaction?

Start with the aromatic aldehyde and acid anhydride. Salt of an Carboxylic acid is used to abstract a proton from the alpha carbon of acid anhydride. The carbanion generated would attack the electrophillic centre of aldehyde. This centre is carbonyl carbon. There after an stablised anion is formed. Further hydrolysis gives the required Cinnamic acid.