another try, declaring only one variable instead of three

iterate_users2.rb

#!/usr/bin/env ruby -wKU

def iterate_users
userlist=`/usr/bin/dscl . -list /users UniqueID`.split("\n").map { |pair| pair.split(" ").to_a }.find_all { |item| item.at(1).to_i > 500 }
p userlist
end

iterate_users