Whenever we use an infinitival phrase to talk about the past, we normally have to use the present perfect:
King Henry VIII is thought (by scientists) to have had a rare
genetic disorder that resulted in a series of miscarriages for two of
his wives.
If we separate the infinitival phrase so that it be in the form of a that-clause, which can only be done by rewording the sentence a wee bit, we can see that the infinitival phrase is, indeed, in the past tense:
Scientists think that King Henry VIII had a rare genetic disorder
that resulted in a series of miscarriages for two of his wives.
If we should choose not to use the present perfect in the infinitival phrase, the meaning is in the present tense; therefore, if I were to say,
King Henry VIII is thought (by scientists) to have a rare genetic
disorder that has resulted in a series of miscarriages for two of
his wives.
I would be, in essence, saying the equivalent of,
Scientists think that King Henry VIII has a rare genetic disorder
that has resulted in a series of miscarriages for two of his
wives.
That sentence above is clearly not talking about something in the past. Now, if I were talking about two past events occurring at the same time, the present perfect in the infinitival phrase wouldn't be correct:
He pretended to be a doctor. = He pretended that he was a
doctor.
However, if the infinitival phrase is used to talk about an even more distant past event than the past event in the main clause, the present perfect is once again used:
Washington, D.C. was once thought to have been built over an
ancient Indian burial ground. = People once thought that Washington,
D.C. had been built over an ancient Indian burial ground.
I hope that might have helped you out. Take care and good luck!
