Thank you ,stevieb and tye. As your suggestions, use multi_param() or scalar param() fix it, there's no warning printed. I would raise a bug report at CPAN, and suggest to modify the example code which makes the reader confused(Since the example returns list context, it should not use 'param()').

"Your name is ",em(scalar param('name')),p,
[download]

or

"Your name is ",em(multi_param('name')),p,
[download]