It would probably be far easier to first verify that the string contains legitimate characters and then check whether or not any of the conditions that would make it illegal exist.

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

for (<DATA>) {
  chomp;
  if (! /[^a-zA-Z0-9,]/ and ! /^,|,{2}|,$/) {
    print "Yep:  $_";
  } else {
    print "Nope: $_";
  }
}

__DATA__
this,should,work,11
,this,should,not,work
this,,too,should,not,work
nor,this,one,
[download]

Please please please read perldoc perlretut and perldoc perlre for a deeper understanding of regular expressions.