Overnight leader Chawalit was tied with Mithun on 17-under heading into the last hole but the Thai set up a three-foot birdie putt with an exquisite approach shot at the par five 18th hole to win his fourth Asian Tour title at the Royal Mingalardon Golf and Country Club.
Chawalit closed with a three-under-par 69 while Mithun, aiming to be the first Sri Lankan to win on the Asian Tour, fought bravely with a sensational 65 at the US$300,000 event, which is the curtain raiser for the 10th Asian Tour season.
There was heartbreak again for Thailand's Kiradech Aphibarnrat, who lost in a play-off last year, as he settled for third following a 70. Hwang In-choon (67) of Korea, Darren Beck (69) of Australia and Angelo Que (70) of the Philippines were a further shot back in fourth.
"I'm happy but a first win in two years is not enough! I want to win more Asian Tour titles. I knew at the 17th hole that Mithun was on 17-under. Luckily, I managed to hole a 15-foot birdie putt (17) to tie for the lead," smiled Chawalit, who registered 18-under-par 270 and won US$54,000.
Chawalit, 38, was a figure of consistency through the week and held the lead for most of the round until Perera came charging home with five birdies in the last seven holes.
"It wasn't smooth for me during the front nine because I was forcing myself to play it safe. I gave myself a bit of pressure and it felt better on the back. This win gives me more confidence to play on the Asian Tour," said the Thai, who was cheered on by his countryman on the 18th hole.
Perera finished 50th on the Asian Tour Order of Merit last year to earn an Asian Tour card for 2013 and his career has been on an upswing since then.
Despite his defeat, the Sri Lankan, who is the son to Nandasena, a famous name on the old Asian circuit during the 1980s and 1990s, remains upbeat of winning his first title on the region's premier Tour.
"All you need is one good week to turn your life around. I think the win is not far away for me. I'm sure my father and my country are proud of me. This week, I flew the Sri Lankan flag high," said the 26-year-old, who was five shots off the lead at the start of the day.
Thai rising star Kiradech cut a forlorn figure as he failed to make amends for last year's play-off defeat. "I played good this week. I couldn't make a putt on the front nine but tried hard. I think the key moment for me was when I birdied 15 to tie for the lead. Again, I couldn't make birdie putts in the last three holes. It was still a good attempt from me."
Darren Beck led the Australians when he finished 4th, his best finish in over two years anywhere.
Scores after round 4 of the ZAYKABAR MYANMAR OPEN being played at the par 72, 7218 Yards RMGcC course (a- denotes amateur):
270 - Chawalit PLAPHOL (THA) 67-66-68-69.
271 - Mithun PERERA (SRI) 71-68-67-65.
272 - Kiradech APHIBARNRAT (THA) 64-67-71-70.
273 - HWANG In-choon (KOR) 66-70-70-67, Darren BECK (AUS) 66-69-69-69, Angelo QUE (PHI) 69-68-66-70.
274 - Quincy QUEK (SIN) 71-69-67-67, Lionel WEBER (FRA) 67-69-70-68, Rahil GANGJEE (IND) 71-66-67-70.
275 - HUNG Chien-yao (TPE) 67-67-71-70, Arnond VONGVANIJ (THA) 68-67-69-71, Chapchai NIRAT (THA) 64-72-67-72.
276 - Rikard KARLBERG (SWE) 67-71-71-67, Antonio LASCUNA (PHI) 67-73-69-67, Himmat RAI (IND) 70-71-67-68, Thanyakon KHRONGPHA (THA) 66-67-74-69, Danny CHIA (MAS) 68-70-68-70, Thammanoon SRIROJ (THA) 69-70-66-71.
277 - Prom MEESAWAT (THA) 65-72-71-69, Tim STEWART (AUS) 71-70-67-69, BAEK Seuk-hyun (KOR) 67-68-72-70.
278 - Berry HENSON (USA) 70-68-68-72, Young NAM (KOR) 66-73-67-72.