Shouldn't make any difference here (as charset is part of the Content-Type header):

$ perl -MCGI -E "say CGI->new()->header(-type => 'text/html', -charset
+ =>'UTF-8')"
Content-Type: text/html; charset=UTF-8
[download]

vs. what the OP had:

$ perl -MCGI -E "say CGI->new()->header(-type => 'text/html; charset=U
+TF-8')"
Content-Type: text/html; charset=UTF-8
[download]

(CGI::App's header_add is eventually passed to CGI.pm's header method)