my $t = XML::Twig->new(
  twig_handlers => {
    'oa:ItemID[@agencyRole="Prefix_Number"]' => \&set_prefix_number,
    'oa:ItemID[@agencyRole="Stock_Number_Butted"]' => \&get_stock_number,
  },
);

$t->parse($xml);

BEGIN {
  my $prefix_number;
  sub set_prefix_number {
    my ($t, $node) = @_;
    $prefix_number = $node->text();
  }

  sub get_stock_number {
    my ($t, $node) = @_;
    my $stock_number = $node->text();
    print "$prefix_number$stock_number\n";
  }
}