You're commenting out @start = (1999,05,27); (which assigns 3 elements to @start) and replacing it with @start = $from; (which assigns 1 element to @start). You later call Delta_Days(@start,@stop); which works when both @start and @stop contain 3 elements each, but doesn't work when both @start and @stop contain only 1 element each.

The solution is to call it with 6 arguments in the form Delta_Days(year1, month1, day1, year2, month2, day2), the fact that the two arguments are scalars which happen to have commas in them doesn't change the fact that they are only two arguments.