The rand() solutions are nearly as predictable, since rand() is deterministic and the latest Perl's seed function is deterministic based on $$, time(), and PL_stack_sp (except on some platforms). That last value, at the time it is checked, is probably often constant across invocations of the same script.

The odds are much worse than 1/10^31 as the entire sequence from rand() is deterministic based on the seed. Perhaps you meant 1/2^32, which presumes a completely random seed, which you don't have.

If you want to avoid predictability, then use Math::TrulyRandom (or encrypt using a secret key, if you think you can keep it secret) as others suggested.

At least Perl has a much better seed generator these days.

Also, even 1/2^31 would be the odds of a single duplicate. If you have lots of IDs with overlapping lifetimes, then the odds rise surprising fast. If you rarely have more than a few IDs active at once, then 1/2^31 might well be acceptable. But if it is possible, for example, to have 6600 simultaneous IDs, you get about a 1% chance of a collisions per batch of IDs; which probably isn't acceptable.

Good point regarding the deterministic sequence by using rand. In that case, bump the number of characters up to 16, or even 32 (which I've seen in a few URLs). Should be much better odds then (it's too early to actually calculate them now :)

No, the number of characters doesn't matter (after a certain point). You could generate 200,000 characters and that wouldn't be any more random or less predictable than if you generated 100 characters because you only have (at most) 2^32 possible seed values and each seed value will produce the exact same sequence, no matter how long you make it.

Update: To be more precise, if selecting from a set of 64 characters, then using 6 characters gives you 2^36 possible strings. So using more than 6 characters is useless if you have only 2^32 possible seed values.

        - tye (but my friends call me "Tye")

my $ID = $ToBase62->( $$ ) . $ToBase62->( rand( $$ ) ) . $ToBase62->( 
+time );
[download]

Update: Don't bother seeding. Tye explains the internal seed here. Also, see the Base Conversion Utility.