#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11148831
use warnings;

sub quarterturn
  {
  my $new = '';
  $new .= "\n" while s/.$/ $new .= $&; '' /gem;
  return $new;
  }

my $forest = do { local (@ARGV, $/) = 'input8.txt'; <> };
my $visible = '';

for ( 0 .. 3 )
  {
  $visible |= $forest =~ s{(.)(?=(.*))}{ $2 =~ /[$1-9]/ ^ 1 }ger;
  $_ = quarterturn for $forest, $visible;
  }

my $totalvisible = $visible =~ tr/1//; # count ones
print "$forest\n$visible\ntotal visible: $totalvisible\n";