use File::Temp;

 my ($FILE, $PATH) = File::Temp::tempfile(DIR => "/some/directory");
 my $res = $ua->request(HTTP::Request->new(GET => $url),
    sub {
       print $FILE $_[0];
       ...update transfer speed statisics...
    });
 close($FILE);
 if ($res->is_success) {
   # determine new name from $res
   rename($PATH, ...new path name...);
 } else {
   unlink $PATH;
 }