Does return from Paypal to website URL require www prefix?

[haprst]

For my website, I configured the Paypal button to return either to https://WebsiteName.com/paid.php or to https://WebsiteName.com/cancelled.php. That didn't work. But when I changed the return URLs to https://www.WebsiteName.com/paid.php and https://www.WebsiteName.com/cancelled.php then it did work. Is this a requirement? If so, why? And why doesn't the documentation explicitly state that requirement?

[sfac]

after transaction is completed by paypal a dollar sign "$" is added as the first character of the receiving program 

example:  is should return to url www.sfac.com/perlprogram.pl but returns instead to url www.sfac.com/$perlprogram.pl

causing a not found message

Can anyone help?