file.rb

require 'json'

data = JSON.load(
"{\"seqno\":2,\"attr\":{\"UID\":1000,\"ENVELOPE\":{\"date\":\"Mon, 1 Jan 2000 00:00:00 +0100\",\"subject\":\"Generic Subject\",\"from\":[{\"name\":\"=?utf-8?q?Generic FullName?=\",\"route\":{},\"mailbox\":\"genericmail\",\"host\":\"generichost.generictld\"}],\"sender\":[{\"name\":\"=?utf-8?q?Generic FullName?=\",\"route\":{},\"mailbox\":\"genericmail\",\"host\":\"generichost.generictld\"}],\"reply_to\":[{\"name\":\"=?utf-8?q?Generic FullName?=\",\"route\":{},\"mailbox\":\"genericmail\",\"host\":\"generichost.generictld\"}],\"to\":[{\"name\":{},\"route\":{},\"mailbox\":\"genericmailbox\",\"host\":\"generichost.generictld\"},{\"name\":{},\"route\":{},\"mailbox\":\"genericmailbox\",\"host\":\"generichost.generictld\"}],\"cc\":{},\"bcc\":{},\"in_reply_to\":{},\"message_id\":\"<GenericMessageID>\"}}}"
)

module EachWithParent
  def each_with_parent(parent=nil, &blk)
    case parent
      when Hash, Struct, OpenStruct
        each do |k, v|
          v.each_with_parent(k, &blk)
        end
      when Array
        map {|v| v.each_with_parent(k, &blk)}
      else
        blk.call(parent)
    end
  end
end

OpenStruct.send(:include, EachWithParent)
Struct.send(:include, EachWithParent)
Hash.send(:include, EachWithParent)
Array.send(:include, EachWithParent)
Object.send(:include, EachWithParent)

data.each_with_parent {|v| puts v.class }  ## nil output