my %seen  = ();
my $carry = "";
my $nextn = 1;

sub nexta {
	my $length = 1;
	my $forbiddenlength = ($carry eq "") ? length($nextn) : 0;

	while (1) {
		if ($length==$forbiddenlength) {
			$length++;
		} elsif (length($carry) < $length) {
			$carry .= $nextn++;
		} elsif ((not exists $seen{ substr($carry, 0, $length) })
			     && (   length($carry)==$length
				     || substr($carry, $length, 1) ne "0")) {
			last;
		} else {
			$length++;
		}
	}

	my $a = substr($carry, 0, $length);
	$seen{$a} = 1;
	$carry = substr($carry, $length);

	return $a;
}

foreach my $n (1..10000) {
	my $a = nexta();
	print "$n $a\n";
}