if ($true_value) {
     system("/usr/home/patch/shelscript.sh");
}

my $counter = 0;
my $start;
my $end;
while (<DATA>) {
   $counter++ if /send/i;
   $start = time if($counter == 1);
   if($counter == 3){
     $counter = 0;
     $end = time;
     system("shellscript.sh &") if(($end - $start) > 3);
   }
}

my @times;
while (<>) {
    next unless /RING/i;        # alles, was kein RING ist, ueberspringen
    if (@times >= 3 && time - shift @times < 3) {   # 3 mal und weniger als 3 Sekunden
            system ('shellskript &') == 0 or warn "kann shellskript nicht starten: $@";
    }
    push @times, time;
}