while ($data_in =~ /<container><a>(.+?)<\/a><b>(.+?)<\/b><\/container>(<\?query\?>)??/ig)  {
  if ( defined $3 ) {
    $data_out = "$2, $1$3 ";
    } else {
    $data_out = "$2, $1 ";
    }
  # further processing
}