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 .....P1p1/6P1/7P/5PK1/8 w - - 4 34"]
  {Weiß am Zug}  *  .....
1n5P/1P4PK/1q6 b - - 2 42"]
  {Weiß am Zug}  *  .....
.......5K2/3R4 w - - 1 33"]
  {Weiß am Zug}  *  .....

my $line = do { local $/;  <>; }; 
$line =~ s/\n/ /g;

...
$line =~ s/(?<=\sb\s)Weiß/Schwarz/g;
...
print $line;