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

my $xml_hash = XMLin('pon01100.xml');
print Dumper($xml_hash);

##</code><code>##

$VAR1 = {
          'reaction' => {
                        'rn:R07892' => {
                                       'substrate' => {
                                                      'name' => 'cpd:C16331',
                                                      'id' => '3038'
                                                    },
                                       'type' => 'reversible',
                                       'id' => '983',
                                       'product' => {
                                                    'name' => 'cpd:C16332',
                                                    'id' => '2201'
                                                  }
                                     },
                        'rn:R02687' => {
                                       'substrate' => {
                                                      'name' => 'cpd:C00641',
                                                      'id' => '3607'
                                                    },
                                       'type' => 'reversible',
                                       'id' => '3606',
                                       'product' => {
                                                    'name' => 'cpd:C01885',
                                                    'id' => '3608'
                                                  }
                                     },
                        'rn:R05640' => {
                                       'substrate' => {
                                                      'name' => 'cpd:C01724',
                                                      'id' => '2269'
                                                    },
                                       'type' => 'reversible',
                                       'id' => '651',
                                       'product' => {
                                                    'name' => 'cpd:C11455',
                                                    'id' => '2270'
                                                  }
                                     }......and so on......