Here is what I use:

my @letters = ('A'..'Z','a'..'z',0..9,qw(! # $ % ^ | _));
my $UID = join("", @letters[map {rand @letters} (1..8)]);
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I don't use the characters @ or & because it is sometimes passed through a cgi and I don't want to make things more confusing than they have to be. By the way, 8 characters should be plenty, as there are about 69^8 possible combinations. Or without the funky characters, 62^8, which is 218340105584896 possible unique IDs. Or get real paranoid and bump it up to 16 characters. :)