What is it to "perish" as the bible speaks in the last days?

 O Jehovah, but let those who love you be like the sun rising in its glory.” (Judg. 5:31) At Armageddon, God’s enemies will likewise "perish", while those who love God will be saved. But there is one important difference between the two conflicts. At Armageddon, God’s people will not fight. They will not even be armed! Their “strength will be in keeping calm and showing trust” in Jehovah and his heavenly armies.​—Isa. 30:15; Rev. 19:11-15.

To perish means to die.

Luke 24:44-48  And he said to them: These are the words which I spoke to you while I was yet with you, that all things must needs be fulfilled which are written in the law of Moses and in the prophets and in the psalms, concerning me. Then he opened their understanding, that they might understand the scriptures. And he said to them: Thus it is written, and thus it behoved Christ to suffer and to rise again from the dead, the third day: And that penance and remission of sins should be preached in his name, unto all nations, beginning at Jerusalem. And you are witnesses of these things. 

In this verse David writes that the sun rises from the end of the heavens, and its circuit takes it to the end of the heavens. Like everyone else, David observed the daily rising of the sun in the east and its setting in the west after making its way through the sky. It completes its journey by returning to its rising place each morning. 

The sun's presence is felt as well as seen. Nothing escapes the light and heat of the sun. Although some places on earth are colder than others, even the coldest places feel its heat. In God's wisdom, He placed the sun at precisely the right distance from the earth - Wherever the sun is visible it offers a testimonial to God's creative wisdom and power. No one can say he or she has received no evidence of God's existence (Psalm 19:1; Romans 1:18–20).

And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever. (Rev: 11:5)

And there shall be no night there; and they need no candle, neither light of the sun; for the Lord God giveth them light: and they shall reign for ever and ever. (Rev:22:5).

1Then the angel showed me a river of the water of life, as clear as crystal, flowing from the throne of God and of the Lamb 2down the middle of the main street of the city. On either side of the river stood a tree of life, bearing twelve kinds of fruit and yielding a fresh crop for each month. And the leaves of the tree are for the healing of the nations.

3 No longer will there be any curse. The throne of God and of the Lamb will be within the city, and His servants will worship Him. 4 They will see His face, and His name will be on their foreheads. 5 There will be no more night in the city, and they will have no need for the light of a lamp or of the sun. For the Lord God will shine on them, and they will reign forever and ever.

Jesus Is Coming

6Then the angel said to me, “These words are faithful and true. The Lord, the God of the spirits of the prophets, has sent His angel to show His servants what must soon take place.

7“ Behold, I am coming soon. Blessed is the one who keeps the words of prophecy in this book.

Isaiah 60:19

No longer will the sun be your light by day, and the brightness of the moon will not shine on you; for the LORD will be your everlasting light, and your God will be your splendor.

Zechariah 14:7

It will be a day known only to the LORD, without day or night; but when evening comes, there will be light.

Psalm 36:9

For with thee is the fountain of life: in thy light shall we see light.

Revelation 21:23

And the city has no need of sun or moon to shine on it, because the glory of God illuminates the city, and the Lamb is its lamp.

Revelation 21:25

Its gates will never be shut at the end of the day, because there will be no night there.

Revelation 21:22-25

And I saw no temple therein: for the Lord God Almighty and the Lamb are the temple of it…

As for this verse you quoted. 

"For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished."  Amen all shall be accomplished, this is the finality of the Law... put to rest... as all is done. Accomplished and satisfied in heaven and in, on the earth... Amen, and amen!

And death shall be no more... we come into the compleat Grace of God, with robes of white linen... crowns, praising God, with the abundance of His promises fulfilled in us who do come! 

They will come and shout for joy on the heights of Zion; they will be radiant in the bounty of the LORD--the grain, the new wine, the fresh oil, and the young of the flocks and herds. Their life will be like a well-watered garden, and never again will they languish. (Jer: 31:12)

And God will wipe away every tear from their eyes; there shall be no more death, nor sorrow, nor crying. There shall be no more pain, for the former things have passed away. (Rev: 21:4)

For the Lamb in the center of the throne will be their shepherd. He will lead them to springs of living water, and God will wipe away every tear from their eyes. (Rev:7:17)

Hosea 13:14

I will ransom them from the power of Sheol; I will redeem them from Death. Where, O Death, are your plagues? Where, O Sheol, is your sting? Compassion is hidden from My eyes.

See all of Rev:21 for further glory.

Rev:22

And the Spirit and the bride say, Come. And let him that heareth say, Come. And let him that is athirst come. And whosoever will, let him take the water of life freely.

18 For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:

19 And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.

20 He which testifieth these things saith, Surely I come quickly. Amen. Even so, come, Lord Jesus.

21 The grace of our Lord Jesus Christ be with you all. Amen.  (Rev:22)

Do some say all will be saved? 

Psalm 84:11

For the LORD God is a sun and shield: the LORD will give grace and glory: no good thing will he withhold from them that walk uprightly.

If so how will this be accomplished? Through Jesus Christ our Lord, The Alpha and Omega, the beginning and the end, the first and the last.

are all the salt of the earth?

Yes... this salt never loose's it's seasoning.. Jesus' Sermon on the Mount: “You are the salt of the earth.” (Matthew 5:13) Jesus meant that the common people he was addressing – fishermen, shepherds, laborers – were worthy and virtuous. He was alluding, not to the tang of salt, but to its value. Like of the grassroots variety, highly favoured...favored, and all who come forth, so, yes! 

Also the Id regarding means… “They shall see His face, and His name shall be on their foreheads.” (Rev: 22:4)

 Rev: 22:21 >>>>  The grace of our Lord Jesus Christ be with you all. Amen.

We need your verse ID regarding "perish." And will check back.

Now then, when Jesus appeared and preached the Gospel, the first Christians were Jews who had been raised under the OT Law. But Jesus began teaching the Grace of God through Jesus Christ as the only means of salvation. God was replacing the OT law with faith in and through Gods Grace. And JESUS IS THE GRACE OF GOD.

So it was a pivotal time in history, and in Gods plan for salvation for all men of faith in Gods Grace.

So many Christian Jews were confused as to how long they should obey the OT law. And Jesus told them .... you keep it UNTIL I HAVE KEPT IT ALL AND FULFILLED IT ALL FOR YOU. Then, and only then, you will be free from the rigorous OT law.

Jesus spent the next 3.5 years keeping (fulfilling) the OT law and prophets.

Luke 24:44 KJV — And Jesus said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.

John 5:46 KJV — For had ye believed Moses, ye would have believed me: for he wrote of me.

Acts 3:22 KJV — For Moses truly said unto the fathers, A prophet shall the Lord your God raise up unto you of your brethren, like unto me; him shall ye hear in all things whatsoever he shall say unto you.

Thats Jesus

John 1:45 KJV — Philip findeth Nathanael, and saith unto him, We have found him, of whom Moses in the law, and the prophets, did write, Jesus of Nazareth, the son of Joseph.

He fulfilled the very last OT item with this event, and then he died on the cross....

John 19:30 KJV — When Jesus therefore had received the vinegar, he said, It is finished: and he bowed his head, and gave up the ghost.

"IT IS FINISHED!"

Jesus had kept the entire law, Psalms and prophets. 

So the crucifixion marked the exact time when the Jewish Christians had been freed from the OT law, and from that time forward, lived under Gods grace through Christ instead.

Paul, a devout Jew, raised under the law as a strict Pharisee, finally learned this....

Colossians 2:14 — Christ blotted out the handwriting of ordinances that was against us, which was contrary to us (the OT LAW) and took it out of the way, nailing it to his cross.

Colossians 2:15-17 — And having spoiled principalities and powers, he made a shew of them openly, triumphing over them in it. Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: Which are a shadow of things to come; but the body is of Christ.

For a person, perish in the Bible doesn't mean cease to exist, and for a person, death in the Bible doesn't mean cease to exist.  Death for a person is to pass onto the next thing.  Perish means ruined forever.  The unbeliever goes to eternal torment.  Nobody has to go to eternal torment.  It is very easy to avoid.  Believe in the Lord Jesus Christ, and you won't go to eternal torment.  Nobody can be good enough to avoid hell.  So to avoid hell, believe in the Lord Jesus Christ.  When a person dies, they will be in heaven or hell, and they can't get out of hell if that is where they go.  To go to heaven and not go to hell, believe in the Lord Jesus Christ, without adding any of your works.  Believe in the Lord Jesus Christ, who is God and who died on the cross and shed His blood to pay for all our sins, and who resurrected from the dead.  See Romans 4:5 and 1 Corinthians 15:3-4.

That verse right there is one of my favorites to quote to legalistic Christians who still eat pig.

It means exactly what it means.