$data = $xml->XMLin($input_file);

foreach $g (@{$data->{reaction}})
{
        foreach $h ($g->{substrate}){
                $temp1 = $h->{name};
                foreach $i ($g->{product}->{name}){
                      $temp2 = $i->{name}
                      print "$temp1 -- $temp2\n";
                }
        ]
}