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";
    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;
[download]

let me try...

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

$newmask is "\xff" where the old and new words differ, that is a new tile must be used. It is "\0" where no tile is to be placed (using existing board state).

    $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 );
[download]

hehehe! GOAL => increment the stack sizes for only the new tiles.

$highs & $newmask produces a string that only has non-null characters where new tiles are to be played. In other words, it isolates the stacks for the new tiles, everywhere else is "\0".
=~ tr/0-4/1-5/r increments the height for each place a new tile is played.
~$newmask inverts $newmask to be "\xff" only where old tiles remain visible.
$highs & ~$newmask produces a string with only the unchanged stack values (and "\0" where the new values are).
Finally, the | puts the old and new stack counts together so they can be replaced in the heights string.

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

This removes the tiles that are used for this word from the string of all tiles, soon to be replaced in @tiles.

Hope this helps...