use warnings;
use strict;


local $/;
my $slurp = do { local $/;  <DATA>; };
print qq{[[$slurp]] \n\n};

my $replace = qq{field2: valueB\n};

my $text = $slurp;

$text =~
s{      ^ field1: [^\n]* \n \K
    (?! ^ \Q$replace\E)  .*?
    (?= ^ field3: [^\n]* \n)
  }
  {$replace}xmsg;
print qq{[[$text]] \n\n};


$text = $slurp;
$text = $text x 100_000;
printf qq{length of timing string: %d \n\n}, length $text;

print qq{begin timings \n};
my $start = time();  # start timing for replacements

$text =~
s{      ^ field1: [^\n]* \n \K
    (?! ^ \Q$replace\E)  .*?
    (?= ^ field3: [^\n]* \n)
  }
  {$replace}xmsg;

printf qq{replacements: %ds elapsed \n}, time() - $start;

$start = time();  # start timing for no replacements

$text =~
s{      ^ field1: [^\n]* \n \K
    (?! ^ \Q$replace\E)  .*?
    (?= ^ field3: [^\n]* \n)
  }
  {$replace}xmsg;

printf qq{no replacements: %ds elapsed \n}, time() - $start;


print qq{\n};
print substr($text, 0, 250), qq{\n... \n}, substr($text, -250), qq{\n};


__DATA__
yada
field1: replace following A
some
lines
here
field3: replace previous A
blah blah
field1: do not replace
field2: valueB
field3: do not replace
field1: replace after B
hoo ha
fee fie foe
field3: replace before B