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sub id2path {
    my $id = shift;
...
    }
    return $path;
}

sub id2path_new {
    my $id = shift or return q();
...
    my $path  = join '/', $id =~ / ^ (.*) ($chunk) ($chunk) $ /xg;
    return $path;
}

use Test::More tests => 23;
is(id2path_new($_), id2path($_), "id=$_") for q(), 0, 1, 9, 10, 99,
...
                                          99999, 100000, 999999, 1e6,
                                          1e7-1, 1e7, 1e8-1, 1e8, 1e9,
                                          1e10, 1e11, 1e12;