while (<DATA>) {
    chomp;
    my ($s, $a, $c, $r) = split /[,\t]/;
    die "Invalid string: $_" if !defined $r;
    print "Processing: $_\n";
}

__DATA__
clientserver,00001,AIT,SOURCE
clientserve|00001|AIT|SOURCE

##</code><code>##

Processing: clientserver,00001,AIT,SOURCE
Invalid string: clientserve|00001|AIT|SOURCE at 1006854.pl line 6, <DATA> line 2.