Legally they arent allowed to lie, but they do. Anyone who has been in court has seen it. You have to have proof they are lying to get them to stop.

That's true of any witness, though. Perjury is a crime, but you have to prove it.

In the trial context, good defense attorneys will be able to introduce contradictory evidence, or will challenge the credibility of a witness by finding facts that make them look bad -- even police officers. With police officers it's a bit harder, though, because the public largely believes (wrongly, I'm afraid) that police are more trustworthy than the general public.

Cops arent just any witness. If they are caught perjuring all the cases they testified against are eligible to be retried.

I have heard this discussed in court as a reason to not find the officer guilty. Records get sealed of course and you cant discuss it.

https://www.innocenceproject.org/prevalence-police-lying/