There are umlauts in that string. When I store this source code file Latin-1 encoded there is one byte per German umlaut. The string is interpreted as a byte string. And this byte string gets interpreted correctly as Perl assumes Latin-1 encoding.
I don't thinks it's entirely accurate to say "Perl assumes Latin-1". It's probably better to say that Perl assumes binary or byte semantics - the bytes from your source code will be output unmodified. If the characters in the source code were in ISO8859-5 Cyrillic it would also "work" in the way you describe.
However, I would recommend that if you have non-ASCII characters in your source code, you should save the source file in UTF8 format and add the use utf8; pragma.