foo.rb

require 'celluloid'

class C
  include Celluloid

  def initialize(&block)
    @block = block
  end

  def call
    @block.call
  end
end

c = C.new { puts 'foo' }
c.call

Output

D, [2013-05-10T13:14:48.846287 #4305] DEBUG -- : Terminating 1 actors...
D, [2013-05-10T13:14:48.846777 #4305] DEBUG -- : Shutdown completed cleanly
/home/tobias/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/2.0.0/thread.rb:72:in `sleep': No live threads left. Deadlock?  from /home/tobias/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/2.0.0/thread.rb:72:in `block (2 levels) in wait'
	from /home/tobias/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/2.0.0/thread.rb:68:in `handle_interrupt'
	from /home/tobias/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/2.0.0/thread.rb:68:in `block in wait'
	from /home/tobias/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/2.0.0/thread.rb:66:in `handle_interrupt'
	from /home/tobias/.rvm/rubies/ruby-2.0.0-p0/lib/ruby/2.0.0/thread.rb:66:in `wait'
	from /home/tobias/.rvm/gems/ruby-2.0.0-p0/gems/celluloid-0.14.0/lib/celluloid/mailbox.rb:73:in `receive'
	from /home/tobias/.rvm/gems/ruby-2.0.0-p0/gems/celluloid-0.14.0/lib/celluloid/calls.rb:100:in `block in wait'
	from /home/tobias/.rvm/gems/ruby-2.0.0-p0/gems/celluloid-0.14.0/lib/celluloid/calls.rb:99:in `loop'
	from /home/tobias/.rvm/gems/ruby-2.0.0-p0/gems/celluloid-0.14.0/lib/celluloid/calls.rb:99:in `wait'
	from /home/tobias/.rvm/gems/ruby-2.0.0-p0/gems/celluloid-0.14.0/lib/celluloid.rb:62:in `suspend'
	from /home/tobias/.rvm/gems/ruby-2.0.0-p0/gems/celluloid-0.14.0/lib/celluloid/calls.rb:95:in `value'
	from /home/tobias/.rvm/gems/ruby-2.0.0-p0/gems/celluloid-0.14.0/lib/celluloid/proxies/sync_proxy.rb:28:in `method_missing'
	from /home/tobias/.rvm/gems/ruby-2.0.0-p0/gems/celluloid-0.14.0/lib/celluloid/legacy.rb:14:in `method_missing'
	from foo.rb:16:in `<main>'