You should look at Apache::Session. You can use it's internal session generation system, without using the entire module:

use Apache::Session::Generate::MD5;

#...

my $session_id = Apache::Session::Generate::MD5::generate();
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You may want to explore this module more. I think it will not only allow you to easily create pseudo-random session id's, but also provide a simple interface to store and retrieve session information. I've used Apache::Session many times and I highly recommend it.