module HashFill
  def fill!(*keys, val)
    raise 'at least one hash key must be listed' if keys.empty?
    nest = self
    keys[0...-1].each do |key|
      nest[key] = {} unless nest[key].is_a? Hash
      nest = nest[key]
    end

    nest[keys.last] = val
  end
end

Hash.include HashFill

h = {}
# {}

h.fill! :foo, :bar, :baz
# :baz
h
# {:foo=>{:bar=>:baz}}