It's very, very rare for anyone to be prosecuted for perjury, and frankly, it's just as well. Eyewitness testimony is inherently very unreliable, and prosecuting someone for giving an account that is obviously wrong could have a serious chilling effect on witnesses, especially in a high profile case like this one. Even a person who actually witnessed an event could be convinced by reading intervening news accounts that it had played out differently than what they had thought.
What has bothered me about McCulloch from the start isn't his personal bias - it's that he has completely abdicated his job. He didn't want to prosecute. Well that's his call to make. So he should have just made it, and taken his lumps. Let the state appoint a special prosecutor over his objections, if they don't like his decision. And if they don't have the political will to do so, well, too bad. Instead, he decided he'd cover his own ass, inflamed the situation, and gave the whole world a front row seat to a biased process.