Holy Pirating, Robin! "Batman" and Warner Bros. have declared war on video piracy. When Warner's new "Batman" film opens June 23, every one of the 4,000 worldwide theatrical prints will bear an identification marking code to enable the tracing of illegally duplicated videocassettes. Warner has used this system on past releases with positive results. The encoded serial marking--not readily visible--is detectable in all pirated film-to-video and subsequent video-to-video transfers, which allows Warner Bros. and Motion Picture Assn. investigators to identify the source of pirated copies. Warner Bros., joined by other film companies, has raided more than 3,799 pirate establishments worldwide--211 in the U.S. alone in the last year. Film piracy may constitute a felony with a maximum penalty of up to five years in prison and/or a $250,000 fine.