Then the sleep() should ensure the pid is not reused within the same tick, so it should still remain unique.

It does depend a bit on the precise sleep() implementation though; possibly safer (and a bit faster) would be:

  my $t = time;
  select(undef, undef, undef, 0.1) while time <= $t;
  $unique = "$t$$";
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It also assumes a monotonically increasing time: if the sysadmin may correct a fast clock backwards, it would be very difficult to cope with. (Note that ntpd will adjust the time in increments of less than a second except when the clock error is "large"; once initially synchronised, a running system corrected by ntpd should never normally skip a full second forwards or backwards.)

Hugo