When you say Response object, I assume you write the app in Perlscript, right or ... Anyway below is a snippet c&p'ed from working code

 
## previous code 
$Response->{'Buffer'} = 1;

$Response->{ContentType} = "application/vnd.ms-excel";
$Response->AddHeader('Content-Disposition', "attachment; filename=$fil
+ename");
#    'Specify size so that the browser's progress bar works properly
$Response->AddHeader( "Content-Length", -s $long_filename);

open(IN, "<$long_filename") || AppendToLog("Unable to open yada yada "
+);
binmode(IN);
my @buf = <IN>;
close(IN);

BinaryWrite(@buf);

$Response->Flush;

## more code


sub BinaryWrite(@) {
    my $output;
    foreach $output (@_) {
        if (length($output) > 128000) {
            BinaryWrite(unpack('a128000 a*', $output));
        } 
        else {
            my $variant =Win32::OLE::Variant->new(VT_UI1, $output);
            $Response->BinaryWrite($variant);
        }
    }
}
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Please also remember that things will fail if you output anything to STDOUT before