Hi Moritz,

Thanks for your reply. However, I still can't get it to work.

I have defined all variables, I use strict and warnings. I have changed the lines for opening the in and out files to

   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: $!";
[download]

because otherwise it gave me this error:

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

With your alterations my program looks like this:

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);
[download]

But this gives me the exact same result as it did without your alterations. Or am I doing something wrong?

Also, I realize I left out one aspect of my program. The substitution only needs to be done when the value of the hash key is 1.

Matje