The “perfected forever” part does not refer to an unconditional eternal security, but refers to the “once for all” sacrifice of CHRIST that saves us – without the need of any repeated future sacrifice for sins. As long as we abide in Him, no more sacrifice has to be made, and we are “perfected forever” through His work- as long as we continue in Him. Nothing can take his offer away or remove us from his love other than our own free will. As for His (Jesus) part in salvation, it has been done ONCE and for all. “Those who are being sanctified” are simply those who are in Christ and abide in Him. It’s all about HIS work in us which WE must allow. We can see that Paul is referring to the number of times Jesus must die on the cross (one) if we read the rest of the context. Paul says:
4For it is not possible that the blood of bulls and of goats should take away sins.–6In burnt offerings and sacrifices for sin thou hast had no pleasure. —9Then said he, Lo, I come to do thy will, O God. He taketh away the first, that he may establish the second.10By the which will we are sanctified through the offering of the body of Jesus Christ once for all.11And every priest standeth daily ministering and offering oftentimes the same sacrifices, which can never take away sins: 12But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God;
If we willfully sin, the only thing left is damnation, unless we repent of course:
18Now where remission of these is, there is no more offering for sin.–23Let US HOLD FAST the profession of our faith without wavering; (for he is faithful that promised;)—26-27 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, But a fearful expectation of judgment and fiery indignation, which shall devour the adversaries.
Since Jesus did his part on the cross, no more sacrifice remains which could save a person. If the sin offering of the son of God was not enough, what other sources are available to save a person? None! So if we despise this offer, only a fearful judgement awaits us.
28He that despised Moses’ law died without mercy under two or three witnesses: 29Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith HE WAS SANCTIFIED, an unholy thing, and hath done despite unto the Spirit of grace?35CAST NOT AWAY therefore your confidence, which hath great recompence of reward.36For ye have need of patience, that, after ye have done the will of God, ye might receive the promise.–38Now the just shall live by faith: but IF ANY MAN DRAW BACK, my soul shall have no pleasure in him.
Read about Hebr. 6:6 here.