#!/usr/bin/perl
use warnings;
use strict;
use XML::Twig;
use feature 'say';

my $parser = XML::Twig::->new(
	twig_handlers => {
		'/pathway/reaction[@name=~/^rn:/]' => \&handle,
                # use XPath to describe what you want to handle
	}
);

$parser->parsefile("myfile");

sub handle {
	my($twig, $elt) = @_;
        # handlers are given an XML::Twig object and an XML::Twig::Elt object
	say $elt->att("name");
	say " substrate[s]: ", map { $_->att("id")." " } $elt->children("substrate");
	say " product[s]: ", map { $_->att("id")." " } $elt->children("product");
}