Short answer: Scoring three goals in a single game. Perfect hat-trick = left foot + right foot + header.
A hat-trick is 3 goals by one player in one match. 'Perfect hat-trick' = one left foot, one right foot, one header. Famous WC final hat-trick: Geoff Hurst, 1966 (only WC final hat-trick ever). Premier League record per season: Erling Haaland holds it. See hat-trick concept page.