use strict;
use warnings;
use XML::Twig;

my $twig = XML::Twig->new(
                          twig_handlers => {
                              offer => sub {
                                  printf("%s,%s,%s\n",
                                         $_->att('id'),
                                         $_->first_child('f1')->text(),
                                         $_->first_child('f2')->text(),
                                        );
                              }
                          }
                         );
$twig->parsefile('a.xml');