my %departments;

while($sth->fetch) {

  $departments{ $department }{department} ||= $department;

  push @{ $departments{ $department }{classes} }, {
    class      => $class,
    department => $department,
    count      => $count,
  };

}

##</code><code>##

$tmpl->param(
  departments => [values %departments],
);

#Note: $tmpl is an HTML::Template object


##</code><code>##

$tmpl->param(
  departments => [@departments{ sort keys %departments }],
);