You've got several separate things going on here and you need to treat them separately. Putting them all together is obviously not working for you, and I can't read your code.

I think you're asking several questions:

• How do I put more than one of the same item in the cart at the same time?
• How do I fetch all of the colors for a product out of the database without getting multiple entries for the product?
• How do I hook those together for the user?

The answer in the first case is to find or make something that uniquely identifies a product. That may be an item id, a product id, or a combination of one id and a color.

The answer in the second case is either to keep a stateful loop or to do two separate queries. I'd probably write code such as this:

sub get_colors_for_item
{
    my ($dbh, item_id) = @_;
    my $sth = $dbh->prepare( 'SELECT color FROM item WHERE item_id = ?
+' );
    $sth->execute( $item_id );
    my @colors;
    while( my ($color) = $sth->fetchrow_array())
    {
        push @colors, $color;
    }
    return @colors;
}
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The answer to your third question is to write much smaller subroutines. Separate the database work from the printing. What's important is to build and return a data structure that makes it easy to do the printing.

I think you're getting caught up on a combinatorial explosion of details. Stop and take a deep breath. Then write some really small bits that do one simple thing at a time. Put them together -- not in one giant subroutine, but by calling them from a parent sub.

It may be an artifact of posting here, but the indentation of this code is really hard to read. perltidy can help. By default, it does a really good job of making things much more readable. You might find that your code is more maintainable that way too.