You iterate over all words, and in each iteration you open the original file, change something, and print the altered output to another file.

This means that the output file is overwritten with what you print in the last iteration.

So you have to change the nesting of the loops. You can get around using the loop over the words altogether with a little trick: 

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 $_;
   }
}

[download]

BTW all of your programs should start like this: 

use strict;
use warnings;

[download]

And you should declare all your variables with either my (most of the time) or our.

In reply to Re: problem with string substitution output by moritz
in thread problem with string substitution output by Anonymous Monk

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