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field1: valueA
field2: valueB
field3: valueC

field1: valueA
some
lines
here
field3: valueC

# this is quick
my $no_field2 = "field1: valueA\nsome\nlines\nhere\nfield3: valueC\n";
$no_field2 .= "........................................\n" x 1000;
...
$has_field2 =~ s/(field1:.*?$)\n(?!^field2:)(.*$)\n(^field3:)
                /$1\nfield2: valueB\n$3
                /msx;