I don't understand the question.

If you use ForceArray => [qw( product color )],

foreach $products (@{$file->{product}})    {
    print "Widget: " . $products->{title} ." \n";
    print "Rating: " . $products->{rating} . "\n";
    print "  Cost: " . $products->{cost} . "\n";
    print "Colors: " . join(', ', @{$products->{color}})."\n\n";
}
[download]

will work for

<shelf>
    <product>
        <title>widget3</title>
        <cost>0.50</cost>
        <rating>S</rating>
        <color>white</color>
    </product>
</shelf>
[download]

and

<shelf>
    <product>
        <title>widget1</title>
        <cost>0.10</cost>
        <rating>B</rating>
        <color>red</color>
        <color>blue</color>
        <color>green</color>
    </product>

    <product>
        <title>widget2</title>
        <cost>0.25</cost>
        <rating>S</rating>
        <color>pink</color>
        <color>gray</color>
        <color>orange</color>
    </product>
</shelf>
[download]

and everything in between. Without ForceArray, you'd need

sub list {
   if (defined(ref($_[0])) && ref($_[0]) eq 'ARRAY') {
      return @{ $_[0] };
   } else {
      return $_[0];
   }
}

foreach $products (list($file->{product}))    {
    print "Widget: " . $products->{title} ." \n";
    print "Rating: " . $products->{rating} . "\n";
    print "  Cost: " . $products->{cost} . "\n";
    print "Colors: " . join(', ', list($products->{color}))."\n\n";
}
[download]

So what's the problem?

Okay. I updated the perl script as follows: prod.pl:

use XML::Simple;
$xml = new XML::Simple( ForceArray => [qw( product color )]);
$file = $xml -> XMLin('./prod.xml', );

foreach $products (@{$file->{product}})    {
    print "Widget: " . $products->{title} ." \n";
    print "Rating: " . $products->{rating} . "\n";
    print "  Cost: " . $products->{cost} . "\n";
    print "Colors: " . (join ", ", @{$products->{color}})."\n\n";
}
[download]

prod.xml

<shelf>
    <product>
        <title>widget1</title>
        <cost>0.10</cost>
        <rating>B</rating>
        <color>red</color>
        <color>blue</color>
        <color>green</color>
    </product>

    <product>
        <title>widget2</title>
        <cost>0.25</cost>
        <rating>S</rating>
        <color>pink</color>
        <color>gray</color>
        <color>orange</color>
    </product>

    <product>
        <title>widget3</title>
        <cost>0.50</cost>
        <rating>S</rating>
        <color>white</color>
    </product>
</shelf>
[download]

However, output with a single item for "color" remains empty:

Widget: widget1
Rating: B
  Cost: 0.10
Colors: red, blue, green

Widget: widget2
Rating: S
  Cost: 0.25
Colors: pink, gray, orange

Widget: widget3
Rating: S
  Cost: 0.50
Colors:
[download]

I don't doubt that ForceArrray works, but apparently I just can't figure out the syntax (ie where to plug it in).

Without changing anything at all, I get:

Widget: widget1
Rating: B
  Cost: 0.10
Colors: red, blue, green

Widget: widget2
Rating: S
  Cost: 0.25
Colors: pink, gray, orange

Widget: widget3
Rating: S
  Cost: 0.50
Colors: white
[download]

Maybe your version of XML::Simple is buggy? Maybe you're not running the script you think you are running? Maybe you're not using the XML file you think you are using?