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

my $next = time() + 0.5;

while( 1 ) {
    sleep .001 while time() < ( $next - 0.01 );
    1 while time() < $next;
    printf "%10.4f\n", time();
    $next += 0.5;
}

__END__
c:\test>849367
1279056441.7570
1279056442.2570
1279056442.7570
1279056443.2570
1279056443.7570
1279056444.2570