[datastore]/P/PE/PER/PerlMonks
[datastore]/P/PE/PER/PerlTheLanguage
[datastore]/F/FL/FLA/FlatFiles

.. in other words ..

my $FileName;
my ($DIR, $DI, $D);
if (length($FileName) >= 3) {
  $DIR = uc(substr($FileName, 0, 3));
  $DI  = substr($DIR, 0, 2);
  $D   = substr($DI, 0, 1);
}

# in case the FileName is 2 characters long (less than 2 not possible)
elsif (length($FileName) == 2) {
  $DIR = uc($FileName) . '0';
  $DI  = uc($FileName);
  $D   = substr($DI, 0, 1);
}