$SIG{ALRM} = \&every_sec();  alarm(1);

open(PROCESS, "tcpdump -ln 'filter'"|);
while(<PROCESS>) {
    next if (!&ParseIPandCheckSomething($_));
    $Counter++;
    }

sub every_sec {
    print "This past second: $Counter\n";
    $Counter = 0;
    alarm(1);
    }