If

• you want to substitute  'Weis' with  'Schwarz' (I will use only latin characters — simpler for me) only in those lines that immediately follow a line with a  'b' in it, and
• (update: a  'b' never occurs in a line that has a  'Weis' you want to change, and)
• you can slurp the entire file into memory at once, and
• your Perl version is 5.10 or greater (for the  \K operator),

here's a way:

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 my $t = qq{ .....P1p1/6P1/7P/5PK1/8 w - - 4 34]\n}
       . qq{  {Weis am Zug}  *  .....\n}
       . qq{1n5P/1P4PK/1q6 b - - 2 42]\n}
       . qq{  {Weis am Zug}  *  .....\n}
       . qq{.......5K2/3R4 w - - 1 33]\n}
       . qq{  {Weis am Zug}  *\n}
       ;
 print qq{[[$t]]};
 ;;
 my $pre  = qr{ \S+ \s }xms;
 my $post = qr{ \s [^\n]+ \n [^\{]+ \{ }xms;
 ;;
 $t =~ s{ ^ $pre b $post \K Weis \b}
        {Schwarz}xmsg;
 print qq{[[$t]]};
"
[[ .....P1p1/6P1/7P/5PK1/8 w - - 4 34]
  {Weis am Zug}  *  .....
1n5P/1P4PK/1q6 b - - 2 42]
  {Weis am Zug}  *  .....
.......5K2/3R4 w - - 1 33]
  {Weis am Zug}  *
]]
[[ .....P1p1/6P1/7P/5PK1/8 w - - 4 34]
  {Weis am Zug}  *  .....
1n5P/1P4PK/1q6 b - - 2 42]
  {Schwarz am Zug}  *  .....
.......5K2/3R4 w - - 1 33]
  {Weis am Zug}  *
]]
[download]

(Also note that I left out all the  " (double-quote) characters — they confuse my REPL.) If your Perl version is pre-5.10, let me know; there's a simple work-around.

Update: If your file is too large to fit entirely in memory and you must process line-by-line in a while-loop, your code might look something like this (untested):

use 5.010;

...

my $pre_b    = qr{ ... }xms;
my $pre_weis = qr{ ... }xms;

my $b_was_seen;

while (my $line = <$input_filehandle>) {

    # check for trigger condition/substitution pattern sequence.
    # ASSUME: 'b' and 'Weis' cannot both occur in same line.
    if ($line =~ m{ \A $pre_b b }xms) {
        # flag 'b' was seen in this line.
        $b_was_seen = 1; 
        }
    elsif ($b_was_seen) {
        # attempt substitution if 'b' seen in previous line.
        $line =~ s{ \A $pre_weis \K Weis }{Schwarz}xms;
        $b_was_seen = 0;
        }
        
    # write each (possibly altered) line to output file.
    print $output_filehandle $line;
    
    }
[download]

(Again, this assumes your Perl version is 5.10+.) The if-test could also be done, perhaps more concisely, with a  .. flip-flop operator (see Range Operators in scalar context in perlop), but verbosity may be a virtue here.