my $chunk_size = 10;
foreach my $chunk (0 .. 10_000 / $chunk_size) {
    my $start = $chunk * $chunk_size;
    my $end   = $start + $chunk_size - 1;

    # I say "${start}" to keep it from being interpreted as $start_
    mkdir "2005/${start}_$end" or die "can't mkdir: $!";

    foreach my $client ($start .. $end) {
        mkdir "2005/${start}_$end/$client" or die "can't mkidir: $!";

        foreach (qw/addendums current archive/) {
            mkdir "2005/${start}_$end/$client/$_" or die "can't mkidir: $!";
        }
    }
}