#!/usr/bin/env perl

use strict;
use warnings;

my $sep = ',-->';
my $sep_len = length $sep;
my $tail = ",\n";
my $last;

while (<DATA>) {
    my ($first, undef, $second) = unpack "A3 A$sep_len A3";
    print $last, $sep, $second, $tail if defined $last;
    $last = $first;
}

__DATA__
123,-->456,
456,-->234,
234,-->789,
789,-->123,

##</code><code>##

123,-->234,
456,-->789,
234,-->123,