Calvin has this to say about Hebrews 5:9, using the word 'sanctified' for 'perfect' :
Sanctified suits the passage better than "made perfect." The Greek word teleiotheis means both; but as he speaks here of the priesthood, he fitly and suitably mentions sanctification. And so Christ himself speaks in another place, "For their sakes I sanctify myself." (John 17:19.) It hence appears that this is to be properly applied to his human nature, in which he performed the office of a priest, and in which he also suffered. 
Gill adds the word 'completed':
And being made perfect,.... In his obedience, through sufferings; having completed his obedience, gone through his sufferings, and finished his sacrifice, and being perfectly glorified in heaven:he became the author of eternal salvation unto all them that obey him
Matthew Henry uses the word 'consecrated' in related to 'perfect' :
(2.) By these his sufferings he was made perfect, and became the author of eternal salvation to all who obey him, v. 9. [1.] Christ by his sufferings was consecrated to his office, consecrated by his own blood. [2.] By his sufferings he consummated that part of his office which was to be performed on earth, making reconciliation for iniquity; and in this sense he is said to be made perfect, a perfect propitiation.