Your while loop is pretty useless.

while (@folder = readdir(DIR)){
    last;
}
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... is basically a convoluted way of writing:

@folder = readdir(DIR);
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Generally speaking, I'd recommend Path::Tiny as a much easier way of dealing with files and directories than Perl's built-ins.

use Path::Tiny 'path';

my $dir = path('/Users/Maxi/Desktop/Verzeichnis beispiel');

for my $file ($dir->children) {
   next if $file->is_dir;
   
   my $fh = $file->openr;
   while (my $line = <$fh>) {
      ...;
   }
}
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