Another, possibly simpler, way might be to split the line on commas then glue it all together again, joining using a comma the first element to the remaining elements that have been joined using an empty string.

knoppix@Microknoppix:~$ perl -E '
> $l = q{abc,def,ghi,jkl,mno};
> say $l;
> @e = split m{,}, $l;
> $l = join q{,}, $e[ 0 ], join q{}, @e[ 1 .. $#e ];
> say $l;'
abc,def,ghi,jkl,mno
abc,defghijklmno
knoppix@Microknoppix:~$
[download]

I hope this is helpful.