for my $top_level (sort keys %$people) {
    next if @{ $people->{$top_level}{parents} };
    print_level($people,$top_level,0);
}

sub print_level {
    my ($people,$name,$level) = @_;
    print "  "x$level,$name,"\n";
    for my $child (@{ $people->{$name}{children} }) {
        print_level($people,$child,$level+1);
    }
}