print "Content-disposition: inline; filename=$filename\n";
In my experience, if you use "inline", the browser will try to show it itself, but if you use "attachment" instead, it'll download it to disk.

Also, I think it's (probably) safer to quote the filename. 

print "Content-disposition: attachment; filename=\"$filename\"\n";

[download]
That's from memory, I haven't rechecked it in practice.

A few online references:

In reply to Re^2: Downloading a file by bart
in thread Downloading a file by JimJx

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