After some experimentation, I have concluded that on my system the resolution of select undef,undef,undef,$fractionalsecond goes down to something on the order of 1/300 of a second, give or take. This is an estimate, and I haven't done proper benchmarks, and YMMV.

 perl -e '
my $times = 5000; print "Starting: " . localtime() . "\n";
for (1..$times) {
   select undef,undef,undef,1/$times;
}
print "Done: " . localtime() . "\n";
'
Starting: Fri Feb 24 10:54:56 2006
Done: Fri Feb 24 10:55:06 2006
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About ten seconds to do 5000 iterations, would lead me to say 500 iterations per second would be the limit (on this system), but if I set $times to 300 it takes a full second or slightly more, so there's some overhead making the calculation imprecise. HTH.HAND.