Hard to guess on that because you've shown no code... and it's not clear whether the display described at the end of your penultimate sentence is something you think is correct (acceptable) or otherwise, so:

... are you sure it's a Perl question, and not:

• ...a problem with PayPal's code?
• ... an html problem (check your headers... both ways!)
            or
• ... (who knows?; need more info!)

Please -- give us a fighting chance to help you... with relevant details, an unambiguous problem statement and your code.