use XML::Twig;

my $twig= new XML::Twig(
    start_tag_handlers => { 'level' => \&start_level },
    twig_handlers      => { 'level' => \&end_level },
);

$twig->parsefile($file);

my @levels;

sub start_level {
    push @levels, $_[1]->{att}{label};
    print join ('.', @levels), "\n" if @levels == 3
}

sub end_level {
    pop @levels;
}