# this is quick
my $no_field2 = "field1: valueA\nsome\nlines\nhere\nfield3: valueC\n";
$no_field2 .= "........................................\n" x 1000;

$no_field2 =~ s/(field1:.*?$)\n(?!^field2:)(.*$)\n(^field3:)
               /$1\nfield2: valueB\n$3
               /msx;

# this is slow
my $has_field2 = "field1: valueA\nfield2: valueB\nfield3: valueC\n";
$has_field2 .= "........................................\n" x 1000;

$has_field2 =~ s/(field1:.*?$)\n(?!^field2:)(.*$)\n(^field3:)
                /$1\nfield2: valueB\n$3
                /msx;