my $t = XML::Twig->new(
  start_tag_handlers => {
    'oa:ItemID' => \&set_agency_role,
  },
  twig_handlers => {
    'us:ItemMasterHeader => \&process_item,
    'oa:ID' => \&item_id,
  },
);

$t->parse($xml);

BEGIN {
  my $agency_role;
  my %id;
  sub set_agency_role {
    my ($t, $node) = @_;
    $agency_role = $node->att('agencyRole');
  }

  sub item_id {
    my ($t, $node) = @_;
    $id{$agency_role} = $node->text();
  }

  sub process_item {
    print "$id{Prefix_Number}$id{Stock_Number_Butted}\n";
  }
}