use XML::LibXML;

my $doc = XML::LibXML->new->parse_fh(\*DATA);
my ( $os ) = $doc->findnodes('//os[text()="Unix"]');
my $content = $os->firstChild();
$content->setData("Solaris");
print $doc->serialize(1);

__DATA__
<xml> <computers> <os>Unix</os> </computers> </xml>

##</code><code>##

<?xml version="1.0"?>
<xml> <computers> <os>Solaris</os> </computers> </xml>