use a method_added callback to realias the methods after the base class version of #perform is defined.

sucker_punch.rb

require 'celluloid'
require 'pry'

module SuckerPunch
  module Job

    def self.included(base)
      base.send(:include, ::Celluloid)

      base.class_eval do
        # set up a callback to SuckerPunch::Job
        # whenever a method is defined on the base class
        def self.method_added(m)
          SuckerPunch::Job.realias_methods(self) if m == :perform
        end
      end
    end

    def perform_with_pool_check(*args, &block)
      define_celluloid_pool
      perform_without_pool_check(*args, &block)
    end

    private

    def self.realias_methods(base)
      # the icky bit prevents an infinite loop
      # otherwise aliasing :perform_with_pool_check to :perform
      # counts will trigger another call to method_added
      if !@icky_bit
        @icky_bit = true
        base.class_eval do
          alias_method :perform_without_pool_check, :perform
          alias_method :perform, :perform_with_pool_check
        end
        remove_instance_variable :@icky_bit
      end
    end

    def define_celluloid_pool
      unless SuckerPunch::Queues.registered?(queue_name)
        Celluloid::Actor[queue_name] = self.class.send(:pool)
        SuckerPunch::Queues.register(queue_name)
      end
    end

    def queue_name
      self.class.to_s.underscore.to_sym
    end

  end
end

class EmailJob
  include SuckerPunch::Job

  def perform(user_id)
    UserMailer.welcome(user_id).deliver
  end
  binding.pry
end