"Perpetual" means continuous or never ending.  And I can understand your concern here.  But it is not a "requirement" at all tax wise, accounting wise or legally.  What is going on is that you are making a contract with the cell tower owner here and they are negotiating that term wit your friend.  They, of course, do not want to invest all that time and money to put up a tower that could become useless to them at some point if their easement is up.  They are not a utility company who has different rights given them by the state and laws to easements on property.  I would suggest that he or she speak with an attorney as to this matter.  Many people do not like the idea of cell towers on their land and this may hurt the resale value so limiting it in tile may be a good idea, if possible.  Good luck.