my $store = $schema->resultset('Items');
$store = restrict_to_department( $store, $department ) if $department;
$store = restrict_to_seller( $store, $seller )         if $seller;
print format_top_ten_items( $store ); # doesn't care whether $store is restricted

sub restrict_to_department {
  $_[0]->search({ 'department.name' => $_[1] }, { join => "departments" });
}