Explanation of the masking in "matchrule"

sub matchrule {
  my ( $old, $highs, $word ) = @_;
  $old eq $word and return 0;
  my $newmask = ( $old ^ $word ) =~ tr/\0/\xff/cr;
  ( $newmask & $highs ) =~ tr/5// and return 0;
  my $tiles = "@tiles";
  $tiles =~ s/$_// or return 0 for ( $newmask & $word ) =~ /\w/g;
  return 1;
}
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Let's follow a call to matchrule('hello', '14222', 'world')

First off, if old and word are the same, it's not a valid move.

Next, get the mask. ( note: strings have been printed by Data::Dumper::Useqq = 1 )

'hello' ^ 'world' => "\37\n\36\0\13"
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However, I want to change all non nulls ("\0") to "\xff" so that characters can pass through these positions unchanged.

my $newmask = ('hello' ^ 'world') =~ tr/\0/\xff/cr => "\377\377\377\0\
+377"
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Now we use this mask against the tile heights and look for any 5's, because the "\xff" in $newmask are at the positions where new tiles will be added.

("\377\377\377\0\377" & '14222') => "142\0002"
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and since there is no 5, the new word is not invalid (yet).

"\377\377\377\0\377" & 'world' => "wor\0d"
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This leaves only the new tiles that must be played, ignoring the "\0" because those positions use the old tile. So remove each new letter from a string of the tile rack, and if any are not there, the move is invalid.

Hope this helps.