By putting a piano key right (3)
'by' is the definition.
'a piano key right' is the wordplay.
'a piano' becomes 'p'.
'key' becomes 'e' (I've seen this in other clues).
'right' becomes 'r' (common abbreviation).
'putting' is the link.
(Other definitions for per that I've seen before include "For each or every", "According to", "Preposition indicating by means of", "Preposition meaning for each".)