It's probably easier to just add something else to your primary key than to try and get sub-second responses. A good example is to use:

$mykey = "$^T$$";
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which sets the key to a combination of the time in seconds and the process ID. This combination will be unique, as long as each time the script is run it generates a single key (e.g. a web script). If the script generates more than one key per second, add a counter:

$mykey = "$^T$$" . $x++;
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If you do need sub-second times, you can look at the Time::HiRes module, although it may not work on your system, as it uses syscall and gettimeofday(2) in C. Probably easier overall to use the technique above.