In any investigation brought by the Attorney General pursuant to this chapter, the Attorney General may grant immunity from prosecution to witnesses called upon to testify therein. No individual granted such immunity shall be excused from attending, testifying or producing documentary material, objects or tangible things in obedience to a subpoena or under order of the court on the ground that the testimony or evidence required of him may tend to incriminate him or subject him to any penalty. No individual granted such immunity shall be criminally prosecuted or subjected to any criminal penalty under this chapter for or on account of any testimony given by him in an investigation brought by the Attorney General pursuant to this chapter; provided no individual so testifying shall be exempt from prosecution or punishment for perjury committed in so testifying.