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$line = $digits[0] * 4**0
      + $digits[1] * 4**1
      + $digits[2] * 4**2

$line = ($digits[2] * 4 + $digits[1]) * 4 + $digits[0]

for my $num_digits (2..3) {
   for my $line (0 .. 4**$num_digits - 1) {
...
      print(join(', ', @digits), "\n");
   }
}

         push @digits, $remain & 3;
         $remain >>= 2;