It wasn't clear if "a client" is just a number, and if you need the three directories made for each of the 10,000 clients.

Also, do you always group together directories in chunks of ten? If the answer to all these is "yes", then something like this may do the trick:

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
+: $!";
        }
    }
}
[download]

(I didn't test this.)