I thought you were asking why there was no separation of the email addresses in the second and third lines of the output shown. Seems like I got that wrong: sorry.

I did go back and studied the question more closely. I note that you've used identical code for both the "prints nothing" and the "prints something":

while (split(/[, ]+/, <DATA>))
{ print @_ }
[download]

I suspect the first block should have "print $_", not "print @_".