What is the meaning of perpetual license on server edition?
I believe it means you are licenced to use the current version forever ie you do not need to pay a licence fee to keep using it next year.
When you buy a server licence it includes 12 months maintenance, which means you can also use any new versions released over the next 12 months forever.
If you want to use new versions releases after 12 months you will need to renew your maintenance. The fee is is discounted by at least half buying the original licence fee.