#!/usr/bin/perl -w
use strict;

die "Usage: ", $0 =~ /([^\/]+)$/, " <numeric_string>\n"
    unless @ARGV && $ARGV[0] =~ /^\d+$/;
my ($c, @list);
for (reverse split //, $ARGV[0]){
    unshift @list, $c++ % 3 || $c == 1 ? $_ : "$_,";
}
print @list;

# Or, instead of "cheating" with an unquoted list, we could
# aggregate to a string. It's not quite as neat, though. :(
#
# my ($c, $out);
# for (reverse split //, $ARGV[0]){
#     $out = $c++ % 3 || $c == 1 ? "$_$out" : "$_,$out";
# }
# print "$out\n";