Do you still think that you have to perform this task with one regular expression? (and a horribly, unreadable, broken one at that.) split is perfectly cabable of stopping after it finds the, say, 3rd element. Then you can do something different with the rest:

use Data::Dumper;

my $foo   = 'one,123,a s d f,a,b,c,bad,e,f,g,h';
my @first = split(',',$foo,4);
my @rest  = split(',',pop @first);

print Dumper \@first, \@rest;

for (0..$#rest) {
   die "index $_ is bad: '$rest[$_]'" if length($rest[$_]) != 1;
}
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Think in chunks. Don't try to swallow the whole pill at once.

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)