sub dump_product {
   my ($product) = @_;
   print(join(', ', @$product), $/) if (@$product);
   @$product = ();
}

my @product;

while (<DATA>) {
   if (/^Product\b/) {
      dump_product(\@product);
   } elsif (/^(\S+)/) {
      push(@product, $1);
   }
}
dump_product(\@product);

__DATA__
Product:
redball This is for Mike.
greenball This is for Dave.
Product:
smallbox This is for apples
bigbox This is for orange
[download]

output:

redball, greenball
smallbox, bigbox
[download]