UPDATE: Reusing such URLs does not make any sense, so ignore this posting :(
I think the easiest solution is just to generate a list with all possible strings - at least for the short ones. Advantages are that you can block strings easily for n days (just add another col in your list or database) and that you always generate the shortest possible string (sort list by string length and availability). If strings should not be reused, then almut's solution is the best.
#!/usr/bin/perl
use strict;
use warnings;
my @alphabet = ((1..9) , qw(a b c));
sub enum_words {
my ($word, $maxlength) = @_;
if (length $word < $maxlength) {
for my $char (@alphabet) {
enum_words($word . $char, $maxlength);
}
}
else {
print "$word\n";
}
}
for my $length (1 .. 4) {
enum_words('', $length);
}