use strict;  
use XML::Simple;  
use Data::Dumper;  
my $xml_hash = XMLin('pon01100.xml');  
print Dumper($xml_hash);

       
$VAR1 = { 
          'reaction' => { 
                         'rn:R01500 rn:R03617' => { 
                                                 'substrate' => { 
                                                                'name' => 'cpd:C00195', 
                                                                'id' => '1815' 
                                                              }, 
                                                 'type' => 'reversible', 
                                                 'id' => '927', 
                                                 'product' => { 
                                                              'name' => 'cpd:C02686', 
                                                              'id' => '2154' 
                                                            } 
                                               }, 
                        'rn:R01001' => { 
                                       'substrate' => { 
                                                      'name' => 'cpd:C02291', 
                                                      'id' => '3509' 
                                                    }, 
                                       'type' => 'reversible', 
                                       'id' => '393', 
                                       'product' => { 
                                                    'cpd:C00097' => { 
                                                                    'id' => '3614' 
                                                                  }, 
                                                    'cpd:C00109' => { 
                                                                    'id' => '1835' 
                                                                  } 
                                                  } 
                                     }, 
                        'rn:R00899' => { 
                                       'substrate' => { 
                                                      'name' => 'cpd:C01419', 
                                                      'id' => '3103' 
                                                    }, 
                                       'type' => 'reversible', 
                                       'id' => '269', 
                                       'product' => { 
                                                    'cpd:C00037' => { 
                                                                    'id' => '1865' 
                                                                  }, 
                                                    'cpd:C00097' => { 
                                                                    'id' => '1959' 
                                                                  } 
                                                  } 
                                     },