{
  my $init = 'aa';
  my $start = 'a';
  my $len = length $init;

  sub next_term {
    my $ret = $init;
    my $p = $len - 1;

    while (1) {
      substr($init, $p, 1) =~ tr/acgnt/cgnta/;
      last if substr($init, $p--, 1) ne $start;
      $init = "$start$init", $len++, last if $p < 0;
    }

    return $ret;
  }
}

##</code><code>##

package Tie::ScalarIter;

sub TIESCALAR {
  my ($class, $init, $start) = @_;
  bless [ $init, $start, length($init) ], $class;
}

sub FETCH {
  my $s = shift;

  my $ret = $s->[0];
  my $p = $s->[2] - 1;

  while (1) {
    substr($s->[0], $p, 1) =~ tr/acgnt/cgnta/;
    last if substr($s->[0], $p--, 1) ne $s->[1];
    $s->[0] = "$s->[1]$s->[0]", $s->[2]++, last if $p < 0;
  }

  return $ret;
}

sub STORE {
  my $s = shift;
  my $rep = shift;

  if (ref $rep) { @$s = (@$rep, length $rep->[0]) }
  else { (@$s[0,2] = ($rep, length $rep) }
}

1;

##</code><code>##

# functional

my @strings;
while (defined (my $next = next_term())) {
  last if length($next) > 4;
  push @strings, $next;
}


# tied scalar
use Tie::ScalarIter;

tie my($iter), 'Tie::ScalarIter', 'aa', 'a';
my @strings;

while (defined (my $next = $iter)) {
  last if length($next) > 4;
  push @strings, $next;
}