c:\@Work\Perl\monks>perl -wMstrict -le
"my $cd     = qr{ cd }xms;
 my $not_cd = qr{ (?! $cd) . }xms;
 ;;
 for my $s (',abcdefg,pqrstuv', ',abefg,pqrstuv', @ARGV) {
   my $t = $s;
   print qq{'$t'};
   $t =~ s{ , $not_cd* ($cd?) [^,]* , }{=$1=}xms;
   print qq{'$t' \n};
   }
"
',abcdefg,pqrstuv'
'=cd=pqrstuv'

',abefg,pqrstuv'
'==pqrstuv'