Australia made 277-8 batting first, with Mike Hussey hitting 67 and Nathan Hauritz 53 not out from 39 balls.
Pakistan were reduced to 39-4, and only 38 from Umar Akmal and 29 from 10 balls from Shahid Afridi allowed them to reach 142 in 37.5 overs.
Harris, who took 5-43 in Adelaide, took 5-19 in only his third ODI.
There is only one game remaining in the series, also in Perth tomorrow.
