use strict;
use warnings;

my $dir = 'C:\Desktop\Plots\Abflusswerte';
opendir my $dh, $dir or die $!;
# keine verzeichnisse
my @files = grep { -f "$dir/$_" } readdir $dh;  
closedir $dh;

open my $out, ">>", "hq_regio_neu.txt" or die $!;

foreach my $file (@files) {
    open my $fh, "<", "$dir/$file" or die $!;
    while (<$fh>) { 
        print $out $_;
    }
}