xml callback

Not sure if I'm posting this in the right place but hopefully there's someone out there that can shed some light on this for me.

I'm using the excellent romancart xml callback function to filter some information into a database on our server. I can extract information and set them as variables from the xml using this format:

$first_name=$rcxml->{'sales-record-fields'}->{'first-name'};
$last_name=$rcxml->{'sales-record-fields'}->{'last-name'};
$address1=$rcxml->{'sales-record-fields'}->{'address1'};

Or:

$order_item=$rcxml->{'order-items'}->{'order-item'}->{'item-name'};
$quantity=$rcxml->{'order-items'}->{'order-item'}->{'quantity'};

Which works fine. However there are a few instances when I specifically want to select the second or third item in an order. I don't want to run them through a foreach loop but I thought I could just target them in this way:

$order_item2=$rcxml->{'order-items'}->{'order-item[1]'}->{'item-name'};
$quantity2=$rcxml->{'order-items'}->{'order-item[1]'}->{'quantity'};
$order_item3=$rcxml->{'order-items'}->{'order-item[2]'}->{'item-name'};
$quantity3=$rcxml->{'order-items'}->{'order-item[2]'}->{'quantity'};

But I get nothing. Can anyone spot the schoolboy error or give me some suggestion as to what I should do to specifically target these elements?

with thanks,

Jem