def _perform(environment)
      val = environment.var(name)
      raise Sass::SyntaxError.new("Undefined variable: \"$#{name}\".") unless val
      if val.is_a?(Sass::Script::Value::Number) && val.original
        val = val.dup
        val.original = nil
      end
      val
    end
def _perform(environment)
      val = environment.var(name)
      raise Sass::SyntaxError.new("Undefined variable: \"$#{name}\".") unless val
      if val.is_a?(Sass::Script::Value::Number) && val.original
        val = val.dup
        val.original = nil
      end
      val
    end
Evaluates the variable.

 @param environment [Sass::Environment] The environment in which to evaluate the SassScript
 @return [Sass::Script::Value] The SassScript object that is the value of the variable
 @raise [Sass::SyntaxError] if the variable is undefined