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    my $id = shift;
    # disallow ID 0;
    $id > 0 && $id < 1e9 or return q();
    $id = sprintf '%0*lu', ($id < 1e6 ? 6 : 9), $id;
    ...

sub id2path {
    my $id = shift;
...
    # or: substr($id, $_, 0, '/') for -2*$n, -$n;
    return $id;
}