c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 my $s = qq{yada\n}
       . qq{field1: valueA\n}
       . qq{some\n}
       . qq{lines\n}
       . qq{here\n}
       . qq{field3: valueC\n}
       . qq{blah blah\n}
       ;
 print qq{[[$s]]};
 ;;
 my $replace = qq{field2: valueB\n};
 ;;
 $s =~ s{
     ^   field1: \s valueA \n \K
     (?! \Q$replace\E)  .*?
     (?= field3: \s valueC \n)
   }
   {$replace}xmsg;
 print qq{[[$s]]};
"
[[yada
field1: valueA
some
lines
here
field3: valueC
blah blah
]]
[[yada
field1: valueA
field2: valueB
field3: valueC
blah blah
]]