I think you will be surprised to find that @localhost when evaluated in double quoted strings actually means "the values found in the ARRAY localhost", which at the point you are using it is 'undef'.

So, all you did was find a new way to specify 'e_shop', which you said does work, as "e_shop(undef)". You have just found a new way to say it the same way. The single quotes were correct and you need to find a different solution.