while ($data_in =~ /<container><a>(.+?)<\/a><b>(.+?)<\/b><\/container>(<\?query\?>)??/ig)  {
  {
    no warnings 'uninitialized';
    $data_out = "$2, $1$3 ";
  }
  # further processing
}

##</code><code>##

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
}

##</code><code>##

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