# slow no longer
my $has_field2 = "field1: valueA\nfield2: valueB\nfield3: valueC\n";
$has_field2 .= "........................................\n" x 1000;

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