.....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}  *  .....
##</code><code>##

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

# if ( $line =~ m/\sb\s/ )
#	{
#		$line =~ s/Weiss/Schwarz/g;
#	}

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