#!/usr/bin/perl

use strict;
use warnings;

package MySAXHandler;
use base qw(XML::SAX::Base);
our $counter = 0;
use Data::Dumper;

sub start_element {
  my ( $self, $el ) = @_;

  if ( $el->{LocalName} eq 'b' ) {
    unless ( ( $el->{Attributes}->{'{}order'}->{Value} || 0 ) != 0 ) {
      $el->{Attributes}->{'{}order'} = {
                                          'LocalName'    => 'order',
                                          'Prefix'       => '',
                                          'Value'        => $counter++,
                                          'Name'         => 'order',
                                          'NamespaceURI' => ''
        },

    }
  }

  # process element start event
  $self->SUPER::start_element($el);
}
1;

use XML::Simple;
use Data::Dumper;
use XML::SAX;
my $xml = <<"ENDE";
<opt>
  <a>
    <b a="12" attr_c="13">test</b>
    <b a="12" attr_c="14">test</b>
    <b a="12" attr_c="15">test</b>
    <b a="12" attr_c="16">test</b>
    <b a="12" attr_c="17">test</b>
  </a>
</opt>
ENDE

my $simple = XML::Simple->new( ForceArray => 1);
my $filter = MySAXHandler->new( Handler               => $simple );
my $parser = XML::SAX::ParserFactory->parser( Handler => $filter );
my $ref    = $parser->parse_string($xml);
print Dumper($ref);