#!/usr/bin/perl
use strict;
use warnings;
my @files = qw(ZIEL1.txt ZIEL2.txt ZIEL3.txt ZIEL4.txt);
my $output = 'ZIELgesamt.txt';
open(my $write_fh, '>', $output) or die $!;
for my $file(@files){
   open(my $fh,'<',$file) or die $!;
   while(my $line = <$fh>){
       print $write_fh $line;
   }
   close $fh;
   }
close $write_fh or die $!;