#!/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>){
        $line =~ tr/\xa0//d;
        print $write_fh $line;
    }
    close $fh;
}
close $write_fh or die $!;