Perin was born and raised in Rohan, the Riddermark. Specifically he is from the western part near Helm's Deep, and Isengard. He grew up tending horses and so met Lormat, one of the fabled Mearas. The two developed a bond, and Lormat allowed Perin to ride him, which was forbidden to those not of the royal house. Perin was caught, and forbiden from having anything to do with Lormat.
In desperation Perin and Lormat fled together. Once free of Rohirrim law, the pair wondered Dunland, and southern Eriador fighting orcs, Dunmen, and bandits. Finally they wondered into Rhudaur, and washed up in Thuin Boid. There they met Sian, who was glad of a fell hand with a sword, so she recruited them.
From the point of view of Sian, and the others they recruited Perin, and "his" horse came with him. From Perin's point of view both he and Lormat agreed together to join the party.
It grieves them both that they cannot return to Rohan, or see their family's or friends again. Never the less they have made new friends. Perin and Matuk both love the outdoors, and are often abroad together when the party is at liesure. Although Matuk is no great hand with a horse, he is getting better. Perin and Lormat still hope that one day, if they gain a sufficient patent of nobility, that they will be allowed to return to Rohan.