my $countdown = 1*60; #60 seconds
$| = 1; #disable output buffering

my $start_time = time;
my $end_time = $start_time + $countdown;


for (;;) {
    my $time = time;
    last if ($time >= $end_time);
    printf("\r%02d:%02d:%02d",
        ($end_time - $time) / (1*60),
        ($end_time - $time) / 60%60,
        ($end_time - $time) % 60,
    );

    sleep(1);
}