Great question, but, you are going to have to put on your thinking cap if you really want to know the answer.
Hebrews teaches a Theme of Perfection, which in the Greek refers to completion, a bringing to an end, rather than our modern concept where we see something as flawless.
Perfection refers primarily to making one complete in regards to what was incomplete in the Old Testament. Christ is the Author and Finisher (Completer) of our faith, and is the One Who makes Complete, and this through His Work.
I will give you an example from the Tenth Chapter, because I think it may be easier to understand the concept there, and will not involve as much Scripture to consider, yet still get the point across:
Hebrews 10:1-4
King James Version (KJV)
1 For the law having a shadow of good things to come, and not the very image of the things, can never with those sacrifices which they offered year by year continually make the comers thereunto perfect.
2 For then would they not have ceased to be offered? because that the worshippers once purged should have had no more conscience of sins.
3 But in those sacrifices there is a remembrance again made of sins every year.
4 For it is not possible that the blood of bulls and of goats should take away sins.
The point is this: the sacrifices offered through the Law could not cease, because they could not take away sin. Perfection is defined here as being remission of sins in totality, where the need for further sacrifice is no longer necessary.
Now, lets look at what can make complete:
Hebrews 10:10-14
King James Version (KJV)
10 By the which will we are sanctified through the offering of the body of Jesus Christ once for all.
11 And every priest standeth daily ministering and offering oftentimes the same sacrifices, which can never take away sins:
12 But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God;
13 From henceforth expecting till his enemies be made his footstool.
14 For by one offering he hath perfected for ever them that are sanctified.
We are made holy, set apart unto God...through the Offering of Jesus Christ of Himself, and this, once ("for all" is not in the original language, it is an insertion).
So what we see in regards to "perfection" is that the sacrifices of the Law could not make one complete in regards to remission of sins, because the blood of bulls and goats could not take away sins.
But the offering of Christ of Himself makes those who have been sanctified by His offering complete in regards to remission of sins forever.
That is why we do not continue to offer up sacrifice. The penalty for our sin has been paid in full.
Now, going back to the verse in question...
Hebrews 12:22-24
King James Version (KJV)
22 But ye are come unto mount Sion, and unto the city of the living God, the heavenly Jerusalem, and to an innumerable company of angels,
23 To the general assembly and church of the firstborn, which are written in heaven, and to God the Judge of all, and to the spirits of just men made perfect,
24 And to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaketh better things than that of Abel.
...this falls within the framework of the New Covenant (which is contrasted throughout the Book with the Covenant of Law), and is not the only place where we see that the Old Testament Saints were not made perfect:
Hebrews 11:13
King James Version (KJV)
13 These all died in faith, not having received the promises, but having seen them afar off, and were persuaded of them, and embraced them, and confessed that they were strangers and pilgrims on the earth.
Hebrews 11:39-40
King James Version (KJV)
39 And these all, having obtained a good report through faith, received not the promise:
40 God having provided some better thing for us, that they without us should not be made perfect.
(Note: I am only notating one occurrence of
teleioō above, see link to see the construction of the statement)
The point of these two verses is to show that the Old Testament Saint received a good report through faith, but, they did not receive the Promises of God, and among those promises was His Promise to forgive sin completely:
Hebrews 10:15-18
King James Version (KJV)
15 Whereof the Holy Ghost also is a witness to us: for after that he had said before,
16 This is the covenant that I will make with them after those days, saith the Lord, I will put my laws into their hearts, and in their minds will I write them;
17 And their sins and iniquities will I remember no more.
18 Now where remission of these is, there is no more offering for sin.
Here we see again that remission of sin was the promise, and that because it is remission of sin in completion (perfection), there is no more offering for sin. And that is precisely what is stated in Hebrew 10:10-14.
So we must not make the mistake of imposing a modern understanding of perfection into the text, but rely on what the original language actually conveyed.
Look at the links, and you will see the definitions and Biblical usage for
telos and its variants. There are other critical passages Perfection is found in, one of them the most debated passage (possibly) in all of Scripture, Hebrews 5:10-6:6.
God bless.
