What is the mechanism behind the formation of phosgene from tetrachloroethylene
Given that the reaction occurrs at high temperatures, the reactive species is likely singlet oxygen, which reacts much like an alkene$^{[1]}$. Electron rich alkenes without allylic hydrogen, like tetrachloroethylene, react with singlet oxygen to form 1,2-dioxetanes$^{[2]}$.



A $[_\pi2_s + _\pi2_a]$ cycloaddition, though a poor reaction geometry for two alkenes, is made more accessible by the small, linear oxygen molecule. This, as well as peroxides' predisposition to nonplanar conformations, makes the transition state hardly any more strained than the dioxetane itself. Once formed, the dioxetane quickly decomposes to give two molecules of phosgene. To give you an idea of their stability:


  A small sample of the [dioxetane] product, on warming to room temperature, melted and exploded.$^{[3]}$





$^{[1]}$ Wikipedia, Singlet Oxygen, Reactions, Organic Chemistry
$^{[2]}$ Singlet Oxygen: Applications in Biosciences and Nanosciences Volume 1
$^{[3]}$ Stereospecific Formation of 1,2-Dioxetanes from cis- and trans Diethoxyethylenes by Singlet Oxygen