$has_field2 =~ s/field1:[\n]*\n\K(?!field2:).*(?=\nfield3:)
                /field2: valueB
                /msx;
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1. Shouldn't the  [\n]*\n after  field1: in the search pattern be  [^\n]*\n (negated class) instead?
2. The  .* in the search pattern is greedy. Your test string has only one occurrence of the  "field1: valueA\nfield2: valueB\nfield3: valueC\n" substring. What happens if you test against many occurrences (see below — BTW, I have an updated version of this script that, among other things, uses hi-res timing if you're interested)? Won't the greedy  .* just gobble all intervening occurrences?
3. You have a newline and a lot of blank space in your replacement string; is this what you want?

   c:\@Work\Perl\monks\Zu>perl -wMstrict -le
   "my $s = 'xxxyxxx';
    $s =~ s/y
           /FOO
           /xms;
    print qq{'$s'};
   "
   'xxxFOO
      xxx'
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