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    my $newmask = ( $old ^ $word ) =~ tr/\0/\xff/cr;
    $flip and ( $board, $heights ) = flip $board, $heights;
...
    my $tiles = join '', @tiles;
    say "word is $word";
    $tiles =~ s/$_// for split //, $word & $newmask;

    my $newmask = ( $old ^ $word ) =~ tr/\0/\xff/cr;

    $flip and ( $board, $heights ) = flip $board, $heights;
    substr $board, $pos, length $word, $word;
...
    #say "new mask is $newmask"; # $newmask has unprintable characters
+, use something like Data::Dump that will show them.
    substr $heights, $pos, length $highs,
      ( $highs & $newmask ) =~ tr/0-4/1-5/r | ( $highs & ~$newmask );

    $flip and ( $board, $heights ) = flip $board, $heights;
    my $tiles = join '', @tiles;
    say "word is $word";
    $tiles =~ s/$_// for split //, $word & $newmask;