Somebody smarter than me will answer in a moment (or has answered while I typed this), but I believe your problem is because the split is assigning to an array ref rather than a real array. More correct (but probably not MOST correct) would be:

/(\S+)\s?=?\s?\[(.+)\]$/  && do {
   @{$hash{$1}} = split /,/, $2;
   next;
};
[download]

(Untested!)

Hot Pastrami