foreach my $iteration (@iterations) {
    print "$iteration\n";  # section header
    my $section = $tree{$iteration};  # array ref
    for my $i (0 .. $#$section) {
        my @data;
        foreach my $name (@names) {
            my $data = $section->[$i]{$name};
            next unless defined $data;
            push @data, "$name:$data";
        }
        if(@data) {
            my $line = join " ", @data;
            print "$line\n";
        }
    }
}