use strict;
use warnings;
use XML::Twig;
my $tw = XML::Twig->new( TwigHandlers => { 'NBA/HomeTeam' => \&nba } );
$tw->parse(<<__XML__);
<NBA> 
<HomeTeam href="Celtics"/> 
<VisitorTeam href="Bulls"/> 
<Stadium id="FleetCenter">blabla</Stadium> 
</NBA> 
__XML__
$tw->flush;

sub nba {
  my ( $twig, $nba ) = @_;
  $nba->set_att( href => 'MyHomeTeam' );
  $twig->flush;
}