Laws regarding hand ball change constantly. 2011 is 8 years ago mate, as much as I wish it would be today.
The interpretation of the rules changed. They were already in effect in this Copa America and Women's World Cup.
The document Marchisio posted is not even the one referees use for training, where they have hundreds of photos and plays from past games to help them to interpret situations.
And the fact is that NOW, when a player does what Silva does, the arm BELOW him/her is considered for supporting the fall. You cant throw yourself to block the ball without having that arm underneath. And NOW it's considered that, thus, it's not a handball anymore. If the ball touched his UPPER arm, it would be a handball.
The referee of the match HAD that training and SAW the photos showing the change of interpretation.
Thus, he made a big mistake awarding that penalty to Peru. Mistakes happen, but what has Messi to say about that? I thought CONMEBOL was dominated by Brazil!!!