open (my $in,"<$file") or die "Can't open file '$file' for reading: $!";
   open (my $out,">$file.out") or die "Can't open file '$file.out' for writing: $!";

##</code><code>##

Unsuccessful open on filename containing newline at test.pl line 18

##</code><code>##

use strict;
use warnings;
my $echo="ECHO";
my $value=undef;
my $key=undef;
my %lijst=();
open (DFFILE,$ARGV[0]) || die "DF-file not found\n";
open (LIST,$ARGV[1]) || die "List not found\n";
while (<DFFILE>) {
	($value, $key) = split(/\t/, $_);
	$lijst{$key} = $value;
}
my @listfiles = <LIST>;
my $all_words_regex = join '|', keys %lijst;
for my $file (@listfiles) {
   open (my $in,"<$file") or die "Can't open file '$file' for reading: $!";
   open (my $out,">$file.out") or die "Can't open file '$file.out' for writing: $!";
   while (<$in>){
       if (m/^($all_words_regex)/){
           my $first_word = $1;
           s/$first_word/$echo/g;
           s/$echo/$first_word/;
       }
       print $out $_;
   }
}
close(DFFILE);