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;