...
    <Entrezgene_unique-keys>
      <Dbtag>
        <Dbtag_db>HGNC</Dbtag_db>
        <Dbtag_tag>
          <Object-id>
            <Object-id_id>5</Object-id_id>
          </Object-id>
        </Dbtag_tag>
      </Dbtag>
      <Dbtag>
        <Dbtag_db>LocusID</Dbtag_db>
        <Dbtag_tag>
          <Object-id>
            <Object-id_id>1</Object-id_id>
          </Object-id>
        </Dbtag_tag>
      </Dbtag>
      <Dbtag>
        <Dbtag_db>MIM</Dbtag_db>
        <Dbtag_tag>
          <Object-id>
            <Object-id_id>138670</Object-id_id>
          </Object-id>
        </Dbtag_tag>
      </Dbtag>
    </Entrezgene_unique-keys>
...

#!/usr/bin/perl -w

use warnings;
use strict;
use DBI;
use XML::Twig; #http://xmltwig.com/xmltwig/tutorial/yapc_twig_s4.html for good references!!!

header();

my $twig = new XML::Twig(
             TwigHandlers =>                     #Gene-track element will be called when each player element has been parsed
               
       {     
               
       "Entrezgene_unique-keys" => \&dbxref
               
       #"Dbtag_tag" => \&dbxref   #funzt nicht, weil es mehere TagElemente diesen Typs gibt
               
       #"Object-id_id" => \&dbxref  #funzt nicht, weil es mehere TagElemente diesen Typs gibt
               
       }
             );

$twig->parsefile("Homo_sapiens.ags");  #built the twig

sub dbxref {
    my ($twig, $dbxref) = @_;

    #leider gehen diese Ansätze nicht, da ich nur das 'erste Kind' erfassen kann. Wie kann ich weitere Subelemente aufrufen?
    my $id = first_child('Dbtag')  #->second_child('Dbtag_db')->text;   
    my $db = first_child('Dbtag')  #->second_child('Dbtag_tag')->third_child('Object-id')->fourth_child('Obejct-id_id')->text; 

    print "$id\t$db\n";
}

...
my $global_db;
my $twig = new XML::Twig(
             TwigRoots =>                        #twig will be created only for Entrezgene_unique-keys elements
             { "Entrezgene_unique-keys" => 1 },             
             TwigHandlers =>                     #Gene-track element will be called when each player element has been parsed
             { 
                 "Dbtag_db" => \&dbxref,           #here $twig->purge forbitten, because the data of the twig are needed for &generef
                 "Object-id_id" => \&dbid
             }
            );
...
sub dbxref {
    my ($twig, $dbxref) = @_;
    
    my $db = '??';
    if ($dbxref->text) {
    $db = $dbxref->text;
    }
    
    $global_db = $db;
    $twig -> purge;
    $dbxref -> delete;    
}

sub dbid {
    my ($twig, $dbid) = @_;
    
    my $id = '??';
    if ($dbid->text) {
    $id = $dbid->text;
    }

    print "$global_db - $id\n";
    $twig -> purge;
    $dbid -> delete;
}

Kopfdaten
Kopfdaten
Kopfdaten
<Klammer>
   <Sub>
     ...
   </Sub>
   <Sub>
     ...
   </Sub>
   <Sub>
     ...
   </Sub>
   <Sub>
     ...
   </Sub>
   ...
</Klammer>