Portland-based filmmaker Irene Taylor Brodsky’s latest documentary, “Beware the Slenderman,” examines the stabbing of a 12-year-old girl, an attack inspired by an internet character, and its aftermath. The film debuts on HBO Jan. 23. On May 31, 2014, in a suburb of Milwaukee, Wisconsin, a 12-year-old girl with multiple stab wounds was found lying on a sidewalk.