Thank you for your help Anomalous Monk, I have applied the substitions you proposed and every thing works fine

use strict;
use warnings;

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

 my %replacement = (
    'b' =>' Schwarz am Zug',
    'w' => 'Weiß am Zug',
    );
 ;;
 my $condition = qr{ \b (?: b | w) \b }xms;
 $text =~ s{
  Event \s+ " \K [?] (?= " .*? FEN [^\n]* \s+ ($condition) \s+ -)
  }
  {$replacement{$1}}xmsg;
 
 print $text;
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