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

',abefg,pqrstuv'
Use of uninitialized value $1 in concatenation (.) or string at -e line 1.
'==pqrstuv'