JMO
I have noticed a trend in trials where hardly ever does a witness who we all suspect is lying to help a defendent hardly ever gets charged with perjury and hardly ever is prosecuted for that charge.
There are many reasons for this and I found a link where a former prosecutor attempted to answer it and they did a good job of explaining the issues surrounding it. Its the first long reply in this link below.
IMO only I think the prosecutor sometimes uses a hostile witness to their advantage too. The prosecutor can sometimes sense the person is lying through their teeth and if its so blatantly obvious then the prosecutor knows the jury will also see that the person is lying and it can actually help the prosecutors case. So they may not pursue that after a trial if the result of the trial satisfied the proscutor's goal anyway.
Another reason IMO is it is a lot of work for the proscutor to try to prove perjury occurred. In order to bring perjury charges, the prosecutor has to be able to prove it by having other different witnesses testify the opposite way and somehow prove their version is the right one. That is no small feat when there is little physical evidence to prove otherwise.
All this and more makes pursuing perjury charges not worth it for a prosecutor unless they can package up a nice compact perjury case against a witness.
https://www.quora.com/Why-are-sworn...ny-is-proven-false-by-the-finding-of-the-jury