Oldie but a goodie, with more of the standard methods.

concurrent_nested_hash.rb

require 'celluloid'

class ConcurrentNestedHash
  include Celluloid
 
  def initialize( starting = {} ); @outer = starting end
 
  def [](*keys); keys.inject(@outer) { |h,k| h[k] } end
 
  def []=(*args)
    value = args.pop
    raise ArgumentError, "wrong number of arguments (1 for 2)" if args.empty?
    key = args.pop
    hash = args.inject(@outer) { |h,k| h[k] ||= {} }
    hash[key] = value
  end

  def has_key?( key ); @outer.has_key? key end
  def merge( h ); @outer.merge h end
  def merge!( h ); @outer.merge! h end
  def delete( key ); @outer.delete key end
  def sort_by( &block ); @outer.sort_by &block end
  def count; @outer.count end
end