I was thinking that using & inside backquotes wouldn't work (and it doesn't make much sense since the caller would have to wait for the completion anyway). I tried it on Cygwin Perl, and at least it works here (using bash for shelling out). I don't know whether this is supposed to work in general.

Another strange thing in your code is that you have

cd $sc `...`
[download]

So provided your backquoted string substitutes to blablabla, you would have something like

cd $sc blablabla
[download]

What is this supposed to do?