Nice code of yours!

I will share an alternative to your regex, which is longer and uses (?{ ... }), but avoids repeated lookahead:

for ( 0 .. 3 )
  {
  my $max;
  $visible |= $forest =~ s
    { ^ (?{ $max = -1; }) (*FAIL) | (\d) }
    { length( $1 > $max and $max = $1 ) }germx;
  $_ = quarterturn for $forest, $visible;
  }
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