Okay, I almost got it to work like this:

use strict;
use warnings;
my $echo = "ECHO";
my ($dffname, $listname) = @ARGV;
my $fh;
my $key=undef;

# read key/val lists
open $fh, '<', $dffname or die "$dffname $!";
my %lijst = map { chomp; (split /\t/)[1,0] } <$fh>;
close $fh;

foreach $key (sort keys %lijst) {
    print "The value associated with key $key is $lijst{$key}\n";}

# readfile list
open $fh, '<', $listname or die "$listname $!";
my @listfiles = <$fh>; chomp @listfiles;
close $fh;

for my $file (@listfiles) {
   # read original file
    open $fh, '<', $file or die "$file $!";
    local $/;  my $content = <$fh>;
    close $fh;
# modify content
    while( my ($key, $val) = each %lijst ) {
        next unless $val == 1;
        $content =~ s/^$key$/$echo/gms;
        $content =~ s/$echo/$key/;
    }
# write modified file
    open $fh, '>', "$file.out" or die "$file.out $!";
    print $fh $content
}
[download]

BUT... in this piece of code:

$content =~ s/^$key$/$echo/gms;
$content =~ s/$echo/$key/;
[download]

the value of the second $key isn't the same as the value of the first $key. The goal of this piece is to first substitute every occurence of $key with $echo, and then to substitute only the first occurence of $echo with $key (thus returning it to it's original value). And that's not working. :( Matje

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