use strict;
use warnings;
use XML::Twig;

my $xmlStr = <<END_XML;
<xml>  <computers>  <os>Unix</os>  </computers> </xml>
END_XML

my $twig = XML::Twig->new (
    twig_roots => {'os' => \&newOs,},
    twig_print_outside_roots => 1,
    );

$twig->parse ($xmlStr);


sub newOs {
    my ($twig, $os) = @_;
    
    $os->set_text ('Solaris');
    $os->print ();
}