my $t = '1,2,3,,5,6,,,9,10,,,,14,15,,,,,,,,,,,,,,,';
my $s = join ',' => map { $_ ne '' ? $_ : '0' }
                    split /,/ => $t, 1 + $t =~ y/,//;
[download]

Update: Or, maybe a bit more readable

my $s = join ',' => map $_ || 0,
                    split /,/ => $t, 1 + $t =~ y/,//;
[download]