Karl Malden as usual refers to someone, in this movie a baseball scout, by the name of Sekulovich. His real birth name.
Anthony Perkins had to bat and throw with his opposite hand to portray the right handed Jimmy Piersall.
During production of this film, Tab Hunter, who had previously starred in the TV production (Fear Strikes Out), dropped by to visit his friend Anthony Perkins on the film set. He got a chilly reception from the other cast members and crew, so he discreetly left. (Source: Autobiography "Tab Hunter Confidential: The Making of a Movie Star" by Tab Hunter and Eddie Muller (2005 - Algonquin Books, Chapel Hill, NC)
Most of Jimmy Piersall's teammates on the Boston Red Sox have unusually high uniform numbers, ranging from the 50s through the 80s. Since numbers that high were rarely used, this move may have been done to avoid associating any real players with the movie.
The real Jimmy Piersall disowned the movie due to its distortion of the facts.
This was a remake of a TV production broadcast live on Aug. 15, 1955 starring Tab Hunter: Fear Strikes Out.