> What's to stop an accused murderer from lying more during the murder trial?
In that case, it would increase the roster of crimes the defendant is accused of. Not so for someone already accused of perjury.
> In a perjury trial, why do you need to ask anything at all?
In a perjury trial, the defendant's attorney might try to argue that the defendant didn't know what he said was false. Perjury requires knowing falsehoods, not inadvertent misstatements of fact. Or the attorney might try to argue that the falsehoods weren't material to the issues being tested in court. There are other defenses -- mental incompetence and others.
In that case, it would increase the roster of crimes the defendant is accused of. Not so for someone already accused of perjury.
> In a perjury trial, why do you need to ask anything at all?
In a perjury trial, the defendant's attorney might try to argue that the defendant didn't know what he said was false. Perjury requires knowing falsehoods, not inadvertent misstatements of fact. Or the attorney might try to argue that the falsehoods weren't material to the issues being tested in court. There are other defenses -- mental incompetence and others.