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>