open(FW_LOG, $firewall_log)
    or die "$firewall_log open() failed: $!";
open(MYNEWFILE, ">>$my_new_log")
    or die "$my_new_log open() failed: $!";
while ($line = <FW_LOG>) {
    if ($line =~ /kernel Temporarily blocking host/ || $line =~ /blocked/) {
        # your line transformations would go here...
        print MYNEWFILE $line;
    }
}
close(FW_LOG) or die "$firewall_log close() failed: $!";
close(MYNEWFILE) or die "$my_new_log close() failed: $!";