my $twig_handlers = {
  q{oa:ItemID[@agencyRole='Prefix_Number']} => \&ITEMnumber
};

##</code><code>##

use strict;
use XML::Rules;

my $parser = XML::Rules->new(
  start_rules => {
    'oa:ItemID' => sub {return $_[1]->{agencyRole} =~ /^(?:Prefix_Number|Stock_Number_Butted)$/}
    # this filters the <oa::ItemID>s we are interested in
  },
  rules => {
    'oa:ID' => 'content',
    'oa:ItemID' => sub {
      return $_[1]->{agencyRole} => $_[1]->{'oa:ID'}
    },
    'us:ItemMasterHeader' => sub {
      print "$_[1]->{Prefix_Number}$_[1]->{Stock_Number_Butted} \n";
      return;
    }
  },
);

$parser->parse(\*DATA);

__DATA__
<us:ItemMaster>
   <us:ItemMasterHeader>
      <oa:ItemID agencyRole="Product_Number">
         <oa:ID>0123456</oa:ID>
      </oa:ItemID>
      <oa:ItemID agencyRole="Prefix_Number">
         <oa:ID>AAA</oa:ID>
      </oa:ItemID>
      <oa:ItemID agencyRole="Stock_Number_Butted">
         <oa:ID>01234</oa:ID>
      </oa:ItemID>
   </us:ItemMasterHeader>
</us:ItemMaster>