sub iterator {
  my($n, $b) = @_;
  my $limit = $b - 1;
  die "base out of range" unless $limit =~ /^\d$/;
  my $str;
  sub {
    return $str = "0" x $n unless defined $str;
    return $str =~ s/([^$limit])($limit*)$/($1+1)."0" x length($2)/e
      ? $str : undef;
  }
}

# Usage:
my $iterator = iterator(5, 2);
while (defined($next = $iterator->())) {
  print "next: $next\n";
}