Perez claimed that Ronaldo has a get-out clause - believed to be around £75million - that allows him to quit United.
Perez all but confirmed that AC Milan star Kaka will join Real in a world record £62m deal.
He said: "The best players in the world, the ones that have won the FIFA World Player of the Year award are Kaka and Cristiano Ronaldo. They're the players Real fans demand.
"We have the will to complete the transfer of Kaka, we're not nervous and I hope in a few days we will happily announce the deal.
"I have said Ronaldo does interest us. We have to resolve this and the other big signings in the month of June.
"In Spain there are get-out clauses. In many cases that eliminates negotiations.
"It seems in the case of Ronaldo he does have a get-out clause and it means if the player brings the money stipulated in the clause he can go.
"It's what I've been told, I've not seen it."