The witness should go to the attorney who requested the witness's appearance and explain the situation before it becomes a larger problem. If the witness has already been charged or found guilty of perjury, it's up to the court as to how it will be handled.
The witness can tell the lawyers or the judge. It should be remembered, however, that perjury is a crime and that it is quite possible that the witness will face criminal charges for his lying under oath.
Perjury is false testimony under oath. The problem with attempting to rectify perjury is that the truth may not be believed. Perjury is a felony and therefore before you do anything, I would consult an attorney so he/she can advise you of your rights.
If you can supply evidence that the witness committed perjury, then the witness will be imprisoned for perjury. From there, if the perjury was pivotal in the verdict, the case could be re-opened and retried.
The witness must immediately notify the judge or prosecutor in the case. The consequences depend on if the perjury was deliberate and how greatly it affected the case.
You may want to get your own lawyer first. Tell your lawyer you want to come clean about your testimony. They will help you notify the proper authorities so you can clear up the matter, & hopefully you can avoid any jail time. Hopefully, no one went to jail already because of this.
