test_actor.rb

require 'celluloid'

class TestActor
  include Celluloid
  
  def initialize
    @handlers = []
  end
  
  def add_handler(&block)
    @handlers << block
  end
  
  def fire
    @handlers.each { |block| block.call }
  end
end

obj = TestActor.new
obj.add_handler do
  puts "Handler 1 fired"
end

obj.add_handler do
  puts "Handler 2 fired"
end

obj.fire

# Deadlock death

You need to use execute_block_on_receiver :add_handler to tell Celluloid to execute the block in the actor context.

See the wiki.
We also have a bug surrounding this.