while ( $sth->fetchrow()  ) {
    push @classes, {
        class      =>      $class,
        department =>      $department,
        count      =>      $count
    };
}

##</code><code>##

@departments = (
    {
        department => "v6"
        classes => [
            {
                class      => "Chemistry",
                department => "v6",
                count      =>  18
            },
            {
                class      => "German I",
                department => "v6",
                count      => 27
            },

           etc ...
        ]
    },
    {
        department => "h4"
        classes => [
            {
                class      => "French II",
                department => "h4",
                count      => 9
            },

            etc ...
        ]
     },

     etc ...
);

##</code><code>##

my %departments;
push @{ $departments{ $_->{department} }{classes} }, $_ foreach (@classes);

my @departments = map {
    $_->{department} = $_->{classes}[0]{department}; $_
} values %departments;