It's probably simpler and clearer (as well as more efficient) to use two regexes for this type of thing.

for ('fred,bill', 'fred,,bill', ',fred', 'bill,') {
    print "Bad:'$_'" unless    
         $_ =~ m[^[a-zA-Z0-9,]+$] and 
         $_ !~ m[,{2}|,$|^,];
}

Bad:'fred,,bill'
Bad:',fred'
Bad:'bill,'
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