Y1 and +1 are used interchangeably to dial US phone numbers from a mobile (more on the plus sign).
When dialing from Peru:
to dial internationally from Peru you have to pick a carrier to complete your call and use its carrier code (Y); here are the main Peru carrier codes: Movistar 1911, Claro 1912, Americatel 1977, IDT 1914, Convergia 1960, Nextel 1990