Hi PaulC,
Thank you for posting your query on Symantec community.
I haven't seen SEP perpetual license in any customer environment. However sales people can provide more details than us.
I have seen 3 years subscibtion max & when you purchase a multi-year license, you receive a set of license files equal to the number of years your license is valid. For instance, a three-year license consists of three separate license files. When you activate a multi-year license, you import all of the license files during the same activation session. The Symantec Endpoint Protection Manager merges the separate license files into a single activated license that is valid for the purchased duration.