Good, but I'd simplify by using a module:

use File::Path;
my $year = 2005;
for my $client ( @clients )
{
    my( $clump ) = $client =~ /(\d*)\d/ or next; # we need digits;
    my $range = $clump.'0_'.$clump.'9';
    mkpath([ map "$year/$range/$client/$_", @subdirs ]);
}
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(Untested)