Because they could have pled the fifth instead of providing the court with a known lie. You assert your innocence by pleading innocent before a trial takes off

Because they could have pled the fifth instead of providing the court with a known lie. You assert your innocence by pleading innocent before a trial takes off. Not by obfuscating on the stand. If an option is to not comment on your innocence under oath, or you'd be lying if you did that, then don't take the stand or remember you don't have to admit to a crime. But unless there are provisions against what lies are applicable to perjury I don't understand how it wouldn't be perjury