use XML::Rules max_docuid => {
  method => 'parse',
  rules => {
    'str' => sub {
      return unless $_[1]->{name} eq 'docuid';
      my $id = $_[1]->{_content};
      $_[4]->{pad} = $id if ($id > $_[4]->{pad});
      return;
    },
    'all' => sub { return $_[4]->{pad}; }
  }
};
#...
print "The max value is: " . max_docuid($xml) . "\n";