Perjury requires you to be giving testimony under oath. This law covers any and all conversations however casual.

Furthermore perjury requires the act to be willful and the item under question to be of material importance to an investigation. This law requires only that the statement be known by the speaker to be incorrect and however trivial.

Perjury is essentially a violation of a contract (the contract being the sworn oath), so of course it's a legitimate offense. To me, it comes down to the context in which you make your remarks. If you're under arrest or under oath, then I think lying should be prosecutable (because, in these situations you have the right to remain silent and the right to not incriminate yourself, and you obviously know you're talking to legal officials).

Lying is a pretty common reaction of some people to stressful situations, especially when they have something to hide. If you are testifying at a trial, you will have had time to collect your thoughts and prepare yourself for the situation, but that isn't the case if an FBI agents shows up at your door and starts asking questions. The law needs to allow for people to be human.