Add this debugging code (replace the current $data = .... line with all this:

my $doc_nr   = param('doc_nr')   || 'undef';
my $doc_name = param('doc_name') || 'undef';
my $author   = param('author')   || 'undef';
my $time     = localtime();

$data = $doc_nr.$doc_name.$author.$time."\n";
[download]

This will work as all elements will now be defined and you will be able to see the problem.

cheers