Perez is usually Spanish Pérez: "son of Pedro," the Iberian form of Peter. The -ez ending does the same patronymic job as in López or Hernández. That reading covers the large majority of US Census Perez households today.
DAFN also notes Breton Pérez and Jewish Peretz (and Sephardic adoption of the Spanish form). Those are real but secondary for the American frequency story. Lead with son of Pedro; check family history before assuming the biblical Peretz line.