$has_field2 =~ s/field1:[\n]*\n\K(?!field2:).*(?=\nfield3:)
                /field2: valueB
                /msx;

##</code><code>##

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