That will depend on how you define 'exact', but on normal operating systems it's not possible to be all that precise.

The help from Time::HiRes says this :-

Remember that unless you are working on a hard realtime system, any cl
+ocks and timers will be imprecise, especially so if you are working i
+n a pre-emptive multiuser system. Understand the difference between w
+allclock time and process time (in UNIX-like systems the sum of user 
+and system times). Any attempt to sleep for X seconds will most proba
+bly end up sleeping more than that, but don't be surprised if you end
+ up sleeping slightly less.
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