Is that a better constructor?

Looks like a much better interface to me ... erm, after you fix the syntax error, by changing:

if (!exists{'payload'}) {
[download]

to:

if (!exists $vars{'payload'}) {
[download]

;-) ... with the caveat that I have zero experience in the CGI domain.

You should also add some new tests with dodgy payloads to verify they're being rejected with a useful error message.