"We are in mourning, so far we have 49 dead," Fedia Castro said on local television, adding that people often travel by truck in this region, because of a shortage of buses.
The victims include 12 children, he added.
Deadly crashes are commonplace in Peru, where poorly maintained roads zig-zag up and down the towering Andes.
In the last week, two other deadly crashes in southern Peru took 19 and 10 lives, respectively.
In the latest accident, the truck fell into an abyss near the town of Suyucuyo and was carrying revelers.
More than 4,000 people were killed in traffic accidents in 2012, according to official statistics.