my %holiday;
while (<DATA>) {
   my (undef, undef, $cc, undef, undef, $date) = split /,/;
   $holiday{$cc}{$date} = 1;
}
delete $holiday{US}{$_} for keys %{$holiday{CA}};
delete $holiday{CA}{$_} for keys %{$holiday{US}};
# Turn into a hash of array:
$holiday{$_} = [sort keys %{$holiday{$_}}] for qw[US CA];