One thing you could do would be to include an identity variable to activate a subroutine/script which updates the text file, without it it simply reads it:

my $check=param('check');
my $value=param('value');

if (($value) && ($check)) {
#update text file here or go to sub routine
} else {
  #show report
  }
[download]

Or something like that...

Update: The check variable would be included in the form submission, and the above code has been revised to reflect that. On discussion with zby this would not work if it is desirable to update the file via the URL - I have assumed it is not. If updates from the URL are required then zby's suggestion would allow this.