Perpetual licenses do not expire. It comes with "Software Assurance" for the first 12 months , which grants access to support and software updates. If "Software Assurance" is not renewed, the license will continue to work although, software updates and support will no longer be available.
Subscription licenses are only valid for 1 year, which includes "Software Assurance". Unlike Perpetual licenses if this is not renewed the license will stop working.