See the link below for a discussion on this. But I think this sums it up: A Court need not accept an oath that would allow a witness to lie with impunity. If the Court wishes to preserve the threat of perjury, the court may compel the witness to acknowledge that he/she is subject to the penalties of perjury. Ferguson v. Commissioner of IRS, 921 F.2d 588 (5th Cir. 1991).

http://www.judges.org/news/news010809.html