...
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;
}