In Perkin's reaction, an aromatic aldehyde reacts with an acid anhydride to give an alpha-beta unsaturated aromatic acid. Which reactant is the electrophile and which one is the nucleophile?
The reaction mechanism shown on the Wikipedia page for the Perkin Reaction is perfectly clear. mechanism here
The first step is deprotonation of acetic anhydride by acetate anion. The second step is nucleophilic attack by the anion of acetic anhydride on the aromatic aldehyde; thus the aldehyde is the electrophile.