#! perl -slw
use strict;
use Time::HiRes qw[ time sleep ];

my $next = time() + 0.5;

while( 1 ) {
    sleep .001 while time() < $next;
    printf "%10.3f\n", time();
    $next += 0.5;
}

__END__
c:\test>849367
1279055103.020
1279055103.520
1279055104.020
1279055104.519
1279055105.020
1279055105.520
1279055106.019
1279055106.520
1279055107.020
1279055107.519
1279055108.020
1279055108.519
1279055109.020
1279055109.519
1279055110.020
1279055110.519
1279055111.020