#$csvIn is a Text::xSV object.
while ($csvIn->get_row()) {
  my %fields = $csvIn->extract_hash();

  # time passes

  # set nulls to empty string.  Have to do this or there will
  # be problems later printing the data rows.
  defined($_) or $_ = "" for values %fields;
}
[download]

The trick for setting nulls to "" might be too cute though. It relies on the way that Perl uses aliasing in a way that might puzzle whoever maintains the code. You might therefore prefer the more verbose:

  for (keys %fields) {
    unless (defined($fields{$_})) {
      $fields{$_} = "";
    }
  }
[download]