use strict;
use warnings;
use DBI;
use XML::Writer;
use Data::Dumper;

my $dbh    = DBI->connect( ... );
my $writer = XML::Writer->new();

$writer->xmlDecl('UTF-8');
$writer->doctype('xml');
$writer->startTag('xml');

my $aa_sql = 'select id,field1,field2,field3 from AA';
my $bb_sql = 'select type, value from BB where id = ?';

for my $row (@{$dbh->selectall_arrayref($aa_sql)}) {
   $writer->startTag('foo',id => $row->[0]);
   $writer->dataElement("field$_",$row->[$_]) for 1..3;

   # warning: i should check for 'bars' before i start a tag
   $writer->startTag('bars');
   for $_ (@{$dbh->selectall_arrayref($bb_sql,undef,$row->[0])}) {
      $writer->dataElement('bar',$_->[1],type => $_->[0]);
   }
   $writer->endTag('bars');
   $writer->endTag();
}

$writer->endTag('xml');
$dbh->disconnect;