while (@folder = readdir(DIR)){
    last;
}

##</code><code>##

@folder = readdir(DIR);

##</code><code>##

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>) {
      ...;
   }
}