use strict;

my $lvl ="@ARGV";
my $out = &idx($lvl);


print "@$out\n";


sub idx {
  my $lvl = shift;
  my @out = qw(a1 b1 c1 d1);

  if($lvl == 1) {
    return \@out;
  } else {
    my @tmp = ();

    for my $o (@out) {
      push @tmp, "$o$_" for(@{&idx($lvl-1)});
    }
    @out = @tmp;
  }

  return \@out;
  
}