Karunaratne and his opening partner Perera reached their fifty partnership in just the seventh over.
London: Sri Lanka made a flying start to their World Cup chase against holders Australia, reaching 122 for one after 17 overs at the Oval on Saturday.
Set 335 to win, Sri Lanka captain Dimuth Karunaratne and Kusal Perera both hit half-centuries in a swashbuckling opening partnership of 115 before Perera was bowled by Mitchell Starc.
Sri Lanka's blistering start gave them a great platform as they attempt to set a World Cup record for the highest successful run chase -- the current record is held by Ireland, who made 329-7 to beat England in 2011.
Karunaratne and his opening partner Perera blasted 24 from the first two overs of their run chase and reached their fifty partnership in just the seventh over.
Perera smoked Jason Behrendorff over cover for a huge six, then survived a caught behind review off Kane Richardson before belting another boundary.
Karunaratne reached him to 50 in 43 balls while Perera only needed 33 balls to reach the mark.
Earlier, Australia captain Aaron Finch equalled his highest one-day international score of 153 as he powered his team to 334-7.
Finch took the misfiring Sri Lanka attack apart with a dazzling display that included 15 fours and five sixes.
"I feel like I'm playing reasonably well. When you hit a few drives early on it gives you some confidence," Finch said.
"They used the big side of the ground really well and it was a bit windy so it was hard to target the small boundary.
"We didn't have a target in mind, we knew their death bowling would be good. We're pretty happy with what we've got, Maxy (Glenn Maxwell) did well at the end."
Finch was well-supported by Steve Smith's 73 from 59 balls and the pair put on 173 runs for the third wicket.
Australia is searching for their fourth win from five matches after beating Afghanistan, the West Indies and Pakistan.
Their lone defeat in the 10-team tournament came against India and a victory over Sri Lanka would put them well on course for a semi-final berth.
Together with opening partner David Warner, Finch took the attack to the Sri Lankans after being put in to bat under cloudy skies in south London.
The pair passed fifty for the opening wicket for the fourth time in five games at the World Cup.
Warner was the first Australian to perish when he was bowled by spinner Dhananjaya de Silva for 26.
Gum-chewing Finch brought up his hundred with a six off Milinda Siriwardana.
Sri Lanka had no answer to his brutal assault and by the time he departed, caught by Karunaratne off Isuru Udana, he had put Australia in a commanding position.
Glenn Maxwell's quick-fire 46 not out capped depressing innings for Sri Lanka, who have one win from four games after their previous two matches were abandoned due to rain.