In a “cronjob” , this “Alarm clock” message is somewhat difficult to hide…
Same effect in bash shell:

$ perl -e 'alarm 5;sleep(10);' >/dev/null 2>&1
Alarm clock

$ ( perl -e 'alarm 5;sleep(10);' ) >/dev/null 2>&1
Alarm clock

My best answer:

( trap 'exit 1' SIGALRM ; perl -e 'alarm 5;sleep(10);' ) >/dev/null

~~READMORE~~

So now, I can write a script like this:

#!/usr/bin/perl -w

use warnings;
use strict;
use IO::Handle;

autoflush STDOUT 1;

{

  alarm(3);

  for( my $i=1; $i<5; $i++ )
  {
    print "Yo! $i\n";
    sleep(1);
  }
  
  print "End never reach.";
}

… and put it in my cronjob like that:

* * * * * ( trap 'exit 1' SIGALRM ; ~/testalarm.pl ) >/dev/null 2>&1

Now: The “Alarm clock” still exists , but it now sent in /dev/null.