Red alert! What's the red pigment in the mouths of the natives you saw in Sarikei and what were they chewing on? Betel leaves and areca nut chewing had been a long tradition in South Asia and S.E. Asia but what drove enthusiasts to stain their mouth? I have never tried this but some articles claimed that this habit stimulates through three angles: an exhilarating lift, a mysterious flavor and a cleansing salivation.
Areca Nut Palm
The ingredients can be easily bought from the markets in Sarikei. Firstly, you must have the areca nut (pinang). The areca nut is sometimes wrongly called the betel nut because it's chewed with the betel leave (sirih). The nut is cut with a tool called pancik.
The areca nut has the alkaloid arecoline, which promotes salivation and is itself a stimulant. The lime (kapur or calcium carbonate) acts to keep the active ingredient in its freebase or alkaline form, thus enabling it to enter the bloodstream via sublingual absorption. (source: wikipedia)
The combination of areca nut and lime wrapped in a betel leave is called a "betel quid" which a person then pops into his mouth to chew and get high.
In the late 1940s and 1950s, people could buy the ingredients for betel quid chewing from No. 4 Repok Road (source: Lidasar). Sometimes tobacco is added as an ingredient in the quid. Now betel quid chewing is no longer as popular as before due to the oral cancer scare.
The local name for betel leave is sirih. There's a chance that Sarikei's old name of Siriki was derived from the betel leave because in the old days, the betel leave was called siriki in the Indian Archipelago. James Brooke and his men may have brought this siriki term from India because he was born in Varanasi, India in 1803. Betel quid chewing was common in the pre-colonial days as documented in many old books written by the colonial folks. Just google for sirih.
Now that's some hypothesis or thought to chew on.