To conjugate modal verbs in the Perfekt, we still need the auxiliary “haben” (conjugated with the subject). The difference here is made by the two infinitives at the end of sentence. That is why we call this “double infinitive”. When the modal verb is associated with another verb in the sentence, we need to form a double infinitive, with the modal verb in the infinitive at the very end, preceded by the other verb in the infinitive.