my @levels;
while (my $row = $sth->fetchrow_hashref) {
    my $level = $row->{level};                     
    $levels[$level]++;
    local $" = ".";
    print "@levels[0 .. $level] $row->{ident}\n";
}
__END__
1 root
1.1 a
1.1.1 b
1.2 c