expire.rb

require "redis"

redis = Redis.new

trap(:INT) {
  redis.unsubscribe
  puts 'bye'
  exit
}

puts redis.set 'hello', 'test', ex: 5

begin
  redis.subscribe('__keyevent@0__:expired') do |on|
    on.subscribe do |channel, subscriptions|
      puts "Subscribed to ##{channel} (#{subscriptions} subscriptions)"
    end

    on.message do |channel, key|
      puts "#{key} has expired"
      redis.set key, "", ex: 2 * (2 ** 3) # does not work here! :(
    end

    on.unsubscribe do |channel, subscriptions|
      puts "Unsubscribed from ##{channel} (#{subscriptions} subscriptions)"
    end
  end
rescue Redis::BaseConnectionError => error
  puts "#{error}, retrying in 1s"
  sleep 1
  retry
end

you'll need to create a different redis object with the #subscribe-block so you can use #set