(A quick aside: The language is Perl, not PERL.)

use strict;
use warnings;

my $countdown = 1*60*60; # in secs.

$| = 1;

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

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

   sleep(1);
}

print("\rKABOOM!!!!\n");
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Below is an alternative, but it will drift. That means that if you count down for 12 hours, it may take 12 hours and 5 minutes, or even much more than that, depending on CPU utilisation of other processes and more. I'm providing it as an example of what you should NOT do.

Bad code:

A minor nit-pick, you need a

$|++;
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in your code, or the timer will not update the display properly.

---

I'm not really a human, but I play one on earth. flash japh

oops, right, you need that on non-Windows systems. I tested my code in Windows, which line-flushes on \r and \n (as opposed to just on \n).

I put it in a simple sub. Easy to adjust if you want to include additional parameters such as a pre or post message.

countdown(10);

sub countdown($) {
    #countdown(seconds);
    my ($duration) = @_;
    my $end_time = time + $duration;
    my $time = time;
    while ($time < $end_time) {
        $time = time;
        printf("\r%02d:%02d:%02d", ($end_time - $time) / (60*60), ($en
+d_time - $time) / (60) % 60,($end_time - $time) % 60); #00:00:10
        $|++;
        sleep 1;
    }
}
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---

countdown(10, "before", "after");

sub countdown($$$) {
    #countdown(seconds, "premessage", "postmessage");
    my ($duration, $premessage, $postmessage) = @_;
    my $end_time = time + $duration;
    my $time = time;
    while ($time < $end_time) {
        $time = time;
        printf("\r%s %02d:%02d:%02d %s", $premessage, ($end_time - $ti
+me) / (60*60), ($end_time - $time) / (60) % 60,($end_time - $time) % 
+60, $postmessage); #before 00:00:10 after
        $|++;
        sleep 1;
    }
}
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1. Calculate how many seconds the countdown should run and store this in $seconds.
2. sleep $seconds;
3. print "Finished"

use strict;
use warnings;

print "How long should I sleep, in hours? ";
my $wait = <STDIN>; ($wait) = $wait =~ /(\d+)/; $wait -= 0;

my $end = time() + $wait * 3600;
sleep 1 while time() < $end;

print "FINISHED";
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