It was one of those pleasant September afternoons when you can feel the heat of summer giving way as the seasons change. At Turner Field in Atlanta, 500 people from more than 90 countries showed up for a solemn yet joyful ceremony: They took an oath and pledged allegiance to the United States as they officially became citizens of the country that is now their home. Eddie Perez, the Atlanta Braves bullpen coach who became a naturalized citizen last year, led them in that pledge.