You need to output an HTML form like this:

print <<HTML;
<form action="https://cart.bamart.com/payment.mart" method="post">
<input type="hidden"  name="ioc_merchant_id" value="$ioc_merchant_id">
<input type="hidden"  name="ioc_order_total_amount" value="30">
.....
<input type="submit" value="Process request">
</form>
HTML
[download]

Thus a user must push a submit button to process a request. If you don't want user interaction, than get rid submit button and insert "onload" javascript hook which will make autosubmitting the form