This is not an example of “complex subject”. It is a passive voice sentence with an infinitive phrase as the complement. The infinitive phrase is also passive, which is kind of cool, but not that unusual. As far as your question, the use of the perfect infinitive is required here in order to place the writing of the book in the past. In your example, the time period mentioned is actually the current century, and the book is still considered now, so of course it should be perfect aspect. That said, even if the time were clearly simple past, we would still use the perfect infinitive, since English doesn’t have a simple past infinitive. (I’m not aware of any language that does, although it’s probable some of the hundreds of languages in the world do.) So, here’s your sentence in the simple past time frame: “The book is considered to have been written in 1976.”

That's not a very natural sentence. Is it one you wrote, or are you basing your question on that as an example?