my @todos = map { { data => $_, level => 1 } } @{$data->{data}};

while (@todos) {
    my $todo = pop @todos;
    print "id: ", $todo->{data}{id}, "\n";
    print "level: ", $todo->{level}, "\n";
    if (exists $todo->{data}{data}) {
        push @todos, map { { data => $_, level => $todo->{level} +1 } } @{$todo->{data}{data}};
    }
}