$tmpl->param( product => $self->dbh->selectall_arrayref(q[SELECT image, price, description, serial FROM product WHERE id=?], { Slice => {} }, $q->param('item') ));

You're selecting image, price, description and serial from the data, and those are going into the products loop as named elements, with their values as was selected.

The problem is, you're using $q->param('item') for your id, but since you're not including that as one of the values that will be available from the SQL, it isn't available for the product loop to use. CGI parameters are not automatically available to templates (though you can easily add them, though I forget the syntax off the top of my head... sorry it's 1am and I don't feel like looking it up).

Something like this will work as well

$tmpl->param( product => $self->dbh->selectall_arrayref(q[SELECT id as "item", image, price, description, serial FROM product WHERE id=?], { Slice => {} }, $q->param('item') ));