This is probably beside the point, but instead of
my @files = File::Find::Rule->file()
->in( '.' );
I would simply use
my @files = <einzelergebnis*.html>;
as all your files seem to be in one directory (which also seems to be your current working directory).
The difference is that your code would also find files that reside in a subdirectories - and that may or may not be what you want. (I would not want subdirs as I then could simply create a subdir and move files I want exclude from processing there but you may think differently about this - you just have to be aware of it).