use strict;
use warnings;

my $base = 4;
my $lvl = shift || 2;

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

sub indices {
  my $lvl = shift;
  die if $lvl < 0;
  my $first = 'A1';
  my @index = map $first, 1..$lvl;
  my @ret;
  my $n = 1;
  {
    push @ret, join '', @index;
    my $p = $n;
    my $j = $#index;
    $index[ $j--] = $first, $p /= $base until $p % $base;
    last if $j < 0;
    $index[ $j ] =
      chr( ord( $index[ $j ] ) + 1 ) . substr( $index[ $j ], 1 );
    ++$n;
    redo;
  }
  return @ret;
}

##</code><code>##

my $glob_string = ( '{' . join( ',', 'A'..'D' ) . '}1' ) x $level;

my @indices;
while ( < $glob_string > ) { push @indices, $_ }