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}  *
]]