punch (someone or something) on (something)
To hit someone or something with one's fist on a particular spot or part. I punched him right on the nose for saying such offensive things. He punched the cabinet on the top part of the door to get it open.
punch someone on something
to strike someone on some body part, typically the shoulder. (Not with the intention of causing harm.) Sally punched Frank on the shoulder just to show they were still friends. Tom punched Fred on the shoulder in a friendly way.