From the docs for sleep (with minor reformatting):

For delays of finer granularity, you may use Perl's syscall interface to access setitimer(2) if your system supports it, or else see select above. The Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) may also help.

Following the reference to select, we find:

You can effect a sleep of 250 milliseconds this way:

select(undef, undef, undef, 0.25);
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Note that whether select gets restarted after signals (say, SIGALRM) is implementation-dependent.

Please try reading the perldocs before asking questions here.