knoppix@Microknoppix:~$ perl -E '
> $l = q{abc,def,ghi,jkl,mno};
> say $l;
> push @p, pos $l while $l =~ m{(?=,)}g;
> substr $l, $_, 1, q{} for reverse @p[ 1 .. $#p ];
> say $l;'
abc,def,ghi,jkl,mno
abc,defghijklmno
knoppix@Microknoppix:~$