I'm not convinced I like any of the above when it comes to readability. Here is what I came up with that might make it easier for a junior-level programmer to at least have half a clue as to what is going on (it being understood that my choice of variable names could be better):

sub id2path_boftx
{
    my $id = shift or return q();

#    my $chunk_length = length $id > 6 ? 3 : 2;
    my $chunk_length = $id < 1e6 ? 2 : 3;
    my $path_pat = '%0' . $chunk_length . 'd';
    $path_pat = join('/', $path_pat, $path_pat, $path_pat);

    my $chunk_power = 10 ** $chunk_length;

    my $path = sprintf $path_pat, $id / ($chunk_power ** 2),
      ($id / $chunk_power) % $chunk_power, $id % $chunk_power;

    return $path;
}
[download]

FWIW, it passes all 23 tests.

Update: Changed how $chunk_lentgh is determined. I like the numeric test better after looking at it for a while.