class Foo
  def proc_method(prc)
    @var = "in proc method"
    self.instance_eval { prc.call }
  end

  def block_method(&blk)
    @var = "in block method"
    self.instance_eval(&blk)
  end
end

foo = Foo.new
prc = proc { @var }

puts foo.proc_method(prc)      # => nil
puts foo.block_method { @var } # => "in block method"