Danushka Gunathilaka, Kusal Mendis and skipper Angelo Mathews shone with the bat to keep the hopes of the 1996 World Cup winners alive.
Fahim Ashraf then marked his one-day worldwide debut by dismissing Dinesh Chandimal for a second-ball duck when he dragged an attempted cover drive on to the stumps.
Dickwella and captain Angelo Mathews stabilised the innings with a 78-run partnership, during which Dickwella brought up his half-century, before Mathews was clean bowled by Amir for 39.
However, Pakistan bowlers hit back and triggered a middle-order collapse before Asela Gunaratne (27) and Suranga Lakmal (26) tried to save the blushes for their side and guide them past 200-run mark.
For Sri Lanka, Nuwan Pradeep sealed three wickets for 60 runs.
Pakistan, especially with the introduction of Junaid Khan as a replacement for the volatile Wahab Riaz alongside Amir and Hasan Ali, now have a triumvirate of pace bowlers who seem adept at exploiting any help.
Mohammad Hafeez (1) fell in a similar manner to Perera a mere over later as he looped a leading edge up to mid-on, while Lakmal claimed the crucial wicket of Azhar, who undid all his earlier hard work by fending a ball well wide of his off-stump to slip.
Shoaib Malik, who was playing in his 250th ODI, fared no better as he was caught behind off the bowling of Lasith Malinga for 11.
Sarfraz finished on 61 off 79 balls, which included five boundaries, while Amir remained undefeated on 28 off 43 deliveries, which included one boundary.
The final league game of the tournament features Pakistan and it takes on Sri Lanka in Cardiff to emerge as the victor of the team.
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Either way, Mathews will be wary of mercurial Pakistan.
Before the nerves got hold of them, Pakistan were cruising.
"I mean, when you lose a game, you can always say it was poor batting, it was poor fielding, and it was poor bowling, but I thought - you know, it was kind of a amusing wicket today".
"We chased down 300-plus runs and that is a good memory", he said.
"Proud of the boys".
"We are now the worst fielding side in Asia, we were never behind Pakistan but now we have gone behind them", Jayaratne told The Nation. Although Sri Lanka has been performing well in the tournament, there is one reason that may cause the team's defeat today.
"I thought we were pathetic once again", he said.
They ve won 11 of their last 12 matches at this level, a far cry from their woeful first-round exit at the 2015 World Cup, and Eoin Morgan s men were the only side to exit the group stage of the Champions Trophy with a flawless played three, won three, record. "Thanks to Amir, he played really well".
"We proved that we can give positive results", he said.