"he was believed to have killed the king but was later acquitted of those charges" - is the perfect infinitive "to have killed" used correctly in this sentence ?
Yes, it is. Here is a definition and example of "believed to be" from Macmillan:
(1) used for saying that it is the general opinion of most people that something is true although it has not been proved
A third man is missing and is believed to have been taken into police custody.
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Yes. But what I really want to know is whether it is also correct if the principal verb(he "was" believed) is in past tense. – Sajjad Hossain Jun 24 '20 at 17:08
Yes, the perfect is valid used that way. See the example at this link to Google books:
Student's Intro to English Grammar "Perfect"
iiia "She is said to have spoken perfect Greek."