my @names = qw(snoopdogg cpu cpu load1);

my $twig = XML::Twig->new;

$twig->parse(<<'XML');
<conf name="snoopdogg" serial="1" target="172.16.1.3">
 <services>
  <service name="cpu">
   <plugins>
    <plugin name="cpu" target="172.16.1.3" schedule="60">
     <probes>
      <probe name="load1">
       <warn value="4" operator="&gt;"/>
       <crit value="10" operator="&gt;"/>
     </probe>
     </probes>
         </plugin></plugins></service></services>
</conf>
XML

my ($elt) = $twig->get_xpath(
  (join "//" => "", map qq{[\@name="$_"]}, @names) . "/warn"
);

printf "value: %d\n", $elt->att('value');

__END__
value: 4