Pink-headed Reed Snake or White-headed Reed SnakeCalamaria schlegeli DUMÉRIL, BIBRON & DUMÉRIL 1854 of south Sarawak / Borneo

Looks like a venomous snake (for example: Bungarus flaviceps or Calliophis bivirgatus) but actually its a non-venomous snake, it might be a form of batesian mimicry.