do{ 
    my $pid = open my $in, "p:/test/dummy.exe |" 
        or die $!; 
    $pid{ $pid }++; 
    printf "\r%7d : $pid    ", scalar keys %pid; 
    print " Reused\n\n" if $pid == 65992 ## The first pid
} for 1 .. 1_000_000;