foo.rb

class Hash
  def true_of?(key, &block)
    TrueOf.new(self, key).instance_eval(&block)
  end
end

class TrueOf
  attr_reader :key, :value

  def initialize(hash, key, &block)
    @hash = hash
    @key = key
    @value = @hash[key]
  end
end

h = {a: 1}
p h.true_of?(:a){ key == :a && value == 1 }