By sorting, you can minimize the calls to mkdir, to open and to close. You can even sort such that the order is preserved in each file if so desired.

#!/usr/bin/perl -w

use strict;

#
# Run the script like:
#    tail -n +2 car.csv | sort | ./foo.pl
#
# Or if you want to preserve order:
#    tail -n +2 car.csv | sort -t, -k1,2 -s | ./foo.pl
#

use File::Path qw( mkpath );

my $out_dir = "/var/tmp/cars";

mkpath($out_dir, 0, 0777) or die;

my $last_make  = '---';
my $last_model = '---';
my $fh;
while (<>) {
   my ($make,$model) = split(/,/, $_, 4);

   if ($make ne $last_make) {
      mkdir("$out_dir/$make", 0777) or die;
      $last_make  = $make;
      $last_model = '---';
   }

   if ($model ne $last_model) {
      $fh = undef;
      open($fh, '>', "$out_dir/$make/$model") or die;
      $last_model = $model;
      $fh = undef;
   }

   print $fh $_;
}
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