Yes. You get an undef whenever the field is null, because you might actually want to store the value 'NULL' in a field for some reason.

If you care to print out a NULL in the case of an undef, you can always do:

while (my $row = $sth->fetchrow_arrayref) {
  foreach my $field (@$row) {
    $field = '(NULL)' unless defined($field);
  }
  ... (the rest of your code) ...
}
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and that should take care of your problem.