Just as what GrandFather was suggesting, this fails badly if $filename has a ' in the name. IF you'd choose a system call instead of the builtin chmod, then at least do it the safe way:

system "chmod", "u+x", $filename;
[download]

If someone cares to set the x bit on a PDF document, it is likely that the document name is littered with unexpected characters.

In the end I'd advice NOT to use system or qx{} to do a simple chmod. Not even to give a fast and simple workaround. I think it is bad advice.