From perlfaq8:

How can I sleep() or alarm() for under a second?

If you want finer granularity than the 1 second that the sleep() function provides, the easiest way is to use the select() function as documented in select in perlfunc. Try the Time::HiRes and the BSD::Itimer modules (available from CPAN, and starting from Perl 5.8 Time::HiRes is part of the standard distribution).