I found your snippet a bit tricky to digest so I've blown some whitespace into it.

sub retrieve {

  my $self = shift;
  use Data::Dumper;
  my $q    = $self->query;
  my $tmpl = $self->load_tmpl(
    "showitem.html",
    loop_context_vars =>1,
    associate => $q
  );
                               
  $tmpl->param(
    product => 
      $self->dbh->selectall_arrayref(
        q[
          SELECT 
            image, 
            price, 
            description, 
            serial 
          FROM 
            product 
          WHERE 
            id = ?
        ], 
        { Slice => {} },
        $q->param('item')
      )
  );
  
  return $frm->build_page(
    { title =>  "Boyo's Place: " . 
        $self->dbh->selectrow_array(
          q[
            SELECT 
              name 
            FROM 
              product 
            WHERE 
              id=?
          ], 
          { Slice => {} }, 
          $q->param('item') 
        ), 
        output => $tmpl->output 
    } 
  ), 
  $q->code(Dumper(\$tmpl));
  
}
[download]

I would use a lot of temp vars to simplify the code and help establish what is happening.

e.g.

my $arrayref = $self->dbh->selectall_arrayref( etc....
die Dumper $arrayref;
# on a subsequent run
my $arrayref2 = $self->dbh->selectrow_array( etc...
die Dumper $arrayref2;
# and even
die Dumper $q->param('item');
[download]

You need to be sure you are feeding your template loop an array of hash refs. Each hash ref will need

item => $item_value
[download]

in it. You may have to loop through the dbh output yourself to put that in.

Hope that helps