Is premium raid pass perpetual? Worth the coins?

No until Niantic changes the earning mechanic.

I believe it is a one use product.
You use it for a raid, and then it disappears...so not perpetual.
It just means you can do a second raid in a day, if you have already used your only pass for the day, and then run into a Legendary raid....but you pay.

It might be handy to keep one in your pack at all times for the time you need it.

You should compare it with two hatches, as this is the price of an additional raid.

So if we know what the reward of a raid will be, we know what to compare this reward with. I assume, i will do additional raids, it they are at least level 3.

You can compare - hatch a new pokemon (buy incubators) or change moveset of an existing pokemon via TM (buy raid-passes)