I followed what I said to you, and here is the code, and result:

use XML::Simple;
use Data::Dumper;

use strict;
use warnings;

my $ref = XMLin("test1.xml");

my $subcats = $ref->{'subcategory'};
for my $subcat (keys(%$subcats)) {
    my $id;
    my $products = $subcats->{$subcat}->{'product'};
    if (ref($products) eq 'ARRAY') {
        $id = $products->[0]->{'product_id'};
        for (0 .. $#{@$products}) {
            delete $products->[$_]->{'product_id'};
        }
    } elsif (ref($products) eq 'HASH') {
        $id = $products->{'product_id'};
        delete $products->{'product_id'};
    }
    $subcats->{$subcat}->{'product_id'} = $id;
}
my $xml = XMLout($ref, KeepRoot=>1, NoAttr => 1);
print $xml;
[download]

<subcategory>
  <Grand Arabica>
    <photo>images/arabica.jpg</photo>
    <product>
      <brand>Legal</brand>
      <channel>A</channel>
      <channel>D</channel>
      <channel>S</channel>
      <conditionning>
        <content>250</content>
        <unit>gr</unit>
      </conditionning>
      <content>Coffee
                </content>
      <description>Grand Arabica</description>
      <packaging_qty>12</packaging_qty>
    </product>
    <product_id>802.55.01</product_id>
  </Grand Arabica>
  <Prestige>
    <photo>images/prestige.jpg</photo>
    <product>
      <brand>Legal</brand>
      <channel>D</channel>
      <channel>A</channel>
      <conditionning>
        <content>250</content>
        <unit>gr</unit>
      </conditionning>
      <content>Coffee
                </content>
      <description>Legal Prestige</description>
      <packaging_qty>12</packaging_qty>
      <recipe>/recipes/coffee1.html</recipe>
    </product>
    <product>
      <brand>Legal</brand>
      <channel>D</channel>
      <channel>A</channel>
      <conditionning>
        <content>500</content>
        <unit>gr</unit>
      </conditionning>
      <content>Coffee
                </content>
      <description>Legal Prestige Boite Arôme</description>
      <packaging_qty>6</packaging_qty>
    </product>
    <product_id>800.55.01</product_id>
  </Prestige>
</subcategory>
[download]