(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");
[download]

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:

use strict;
use warnings;

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

while ($countdown--) {
   printf("\r%02d:%02d:%02d",
      $time_left / (60*60),
      $time_left / (   60) % 60,
      $time_left           % 60,
   );

   sleep(1);
}

print("\rKABOOM!!!!\n");
[download]