use strict;
use warnings;

my $lvl = shift || 2;

my @indices = indices( $lvl );
print "$_\n" for @indices;

sub indices {
  my $lvl = shift;
  die if $lvl < 0;
  return $lvl ? map make_indices( $_ ), indices( $lvl - 1 ) : ( '' );
}

sub make_indices {
  my $base = shift;
  return map "$base${_}1", 'A' .. 'D';
}