use XML::Rules;

XML::Rules->new(
    stripspaces => 7,
    rules => {
        lease => sub {return 'lease' => $_[1]->{true}},
        extra_device => sub {return 'extra_device' => $_[1]->{value}},
        computer => sub {
            if ($_[1]->{extra_device}) {
                print "computer id: $_[1]->{id} type: $_[1]->{type} os
+: $_[1]->{os} lease:$_[1]->{lease} extra_device=$_[1]->{extra_device}
+\n";
            } else {
                print "computer id: $_[1]->{id} type: $_[1]->{type} os
+: $_[1]->{os} lease:$_[1]->{lease}\n";
            }
            return;
        }
    }
)->parse(\*DATA);

__DATA__
<config>
  <computer id="one" type="mac" os="XP" >
        <lease true="yes" />
        <extra_device value="scanner"/>
  </computer>
  <computer id="two" type="pc" os="NT" >
        <lease true="no" />
  </computer>
</config>
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