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 $_;
   }
}

##</code><code>##

use strict;
use warnings;