A guess: you have an alias called 'perl' that invokes perl.exe on the first argument. Unforunately, you don't pass any other arguments. I'd expect to see something like this (though it's untested):

/path/to/perl.exe %1 %2 %3 %4 %5 %6 %7 %8 %9
[download]

(Disclaimer: that looks inelegant even to me, and I'm not a great Windows programmer.)