use strict;
use warnings;
use XML::Simple;

my $xml = '<xml>  <computers>  <os>Unix</os>  </computers> </xml>';
my $ref = XMLin($xml);
$ref->{computers}{os} = 'Solaris';
$xml = XMLout( $ref, RootName => 'xml', NoAttr => 1, NoIndent => 1 );
print "$xml\n";

__END__
<xml><computers><os>Solaris</os></computers></xml>