Can't you redirect STDERR to a log file? ie call the perl script in the batch file with:

perl script.pl 2>>error.log
[download]

That should give you something to go on.

I know next to nothing about dos/windows, so this might not work. try it. otherwise put

BEGIN {
 open STDERR,">>","error.log" or die "Can't open error.log: $!";
}
[download]

at the top of the script.