If I understand your intentions correctly, you mean to append the contents of a second file to the end of the output stream provided by the uploaded file. If so, there are a few different ways of arriving at such a result. Here's one of them that doesn't require anything fancy.

my $q = CGI->new();

my $upload = $q->upload('upload');
open(my $second, '<', 'second_file.dat') or die($!);

open(my $out, '>', 'output.dat') or die($!);
for my $fh ($upload, $second) {
    print $out $_ while <$fh>;
}
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