Yet another way would be to use substr working back from the right end of the string.

knoppix@Microknoppix:~$ perl -E '
> $str = q{ABCDEFG};
> substr $str, $_, 0, q{,} for reverse 1 .. length( $str ) - 1;
> say $str;'
A,B,C,D,E,F,G
knoppix@Microknoppix:~$
[download]

I hope this is useful.