using Kühne in a string is using utf-8 in the code?

It is, if it is written with utf-8 encoding. use utf8; tells perl that your source code includes literal utf-8 characters. If you don't include that but insert literal utf-8 characters in your code, all manner of bad things will ensue. See utf8.

There is almost no reason not to use uft8; in all your code these days.