use strict;
use warnings;

use XML::Simple;
use Data::Dumper;

local $/; # slurp

my $data = XMLin(<DATA>, KeyAttr => [], ForceArray => [qw/substrate product/]);

for my $reaction (@{$data->{reaction}}) {
    print $reaction->{name}, ' (', $reaction->{type}, "):\n";
    for my $substrate (@{$reaction->{substrate}}) {
        foreach my $product (@{$reaction->{product}}) {
            print "\t", $substrate->{name}, " -- ", $product->{name}, "\n";
        }
    }
}

__DATA__
<root>
    <reaction name="rn:R00710" type="reversible">
        <substrate name="cpd:C00084"/>
        <product name="cpd:C00033"/>
    </reaction>
    <reaction name="rn:R00014" type="irreversible">
        <substrate name="cpd:C00068"/>
        <substrate name="cpd:C00022"/>
        <product name="cpd:C05125"/>
    </reaction>
</root>