Is it true as Perseus rode Pegasus over a desert the blood of the venemous snakes from medusa's head dropped on the desrt and made the desert full of venemous snakes?

The story is actually the Medusa's blood, not that of the snakes, was poisonous. The rest is accurate, though. As Perseus flew over the deep desert of the Sahara, the drops of blood from Medusa brought forth the dangerous vipers known in the region.

Incidentally, her blood is also credited with created the coral in the Red Sea.