my $t = do { local $/;  <>; }; 

my $pre  = qr{ \S+ \s }xms;
 my $post = qr{ \s [^\n]+ \n [^\{]+ \{ }xms;
 ;;
 $t =~ s{ ^ $pre b $post \K Weiss \b}
                {Schwarz}xmsg;
 
 print qq{$t};