Is it obligatory to make up the fast for one who tasted a chemical substance (perfume) then spat it out, and doubted the presence of some contaminated saliva on the inside of the lips?

If a fasting person did not intentionally swallow saliva mixed with the taste of a substance, then their fast is valid, and they are not required to make up for it. It is recommended to ignore intrusive thoughts (waswas) and not to dwell on them.