$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);
    }

##</code><code>##

foreach $filter (qw(filter1 filter2 filter3)) {
    my $fh = FileHandle->new();
    $fh->open("tcpdump -ln $filter|");
    $fh->autoflush(1);
    push(@FHStack, $fh);
    }

while(1) {
    # Read anything (up to 4096 bytes) from tcpdump
    foreach my $fh (@FHStack) {
         my $bytes = read($FH, my $buffer, 4096);
         $PackStack{$FH} .= $buffer;
         }

    # Process what we've gotten, looking for a complete packet
    foreach my $fh (@FHStack) {
         if ($PackStack{$FH} =~ /\n/) {
              # LF means completed packet received from tcpdump
              $PackStack{$FH} = &ProcessPacketAndRemove($PackStack{$FH});
              }
         }
     }