If your goal is to create unguessable unique keys and you don't care about leaking info about the order in which the key are created, you could concatenate a unique incrementing number with a random number.

my $unique = time;
my $key_bin = pack('N', $unique)
            . pack('C*', map { int(rand()*256) } 1..6);
my $key_hex = unpack('H*', $key_bin);
[download]

That gives something like

4818e14c1662fd0dac98
\______/\__________/
   |         |
 32-bit    48-bit
 unique    random
[download]

You can reduce the size of the unique portion (by using substr(pack('N', $unique), 1)) and change the size of the random portion (by changing the "6") at will.