Question: I've a question related to a very basic thing in Perl, but I'm unable to find an efficient solution.

Here's a bit of context first. I use [Net::Pcap](http://search.cpan.org/perldoc/Net::Pcap) etc and when I'm in my function which processes packets (used by `pcap_loop`) I get a `$packet` scalar which contains my whole packet (ethernet header + ip header + tcp/udp header + payload).

What I want to do is to change the first 6 bytes of this `$packet` (the ethernet destination) in order to get a `$packet` that I can send using `pcap_sendpacket`, using a user defined destination mac address (for example passed as a command line argument or so), such as `00:11:22:33:44:55`. So I can split the user defined address (using `split` for example) to get every 6 parts of the destination mac address, and convert it to hex using the `hex` function, but now I want to modify the first bytes of my packet to replace them with these `hex`ed bytes. How should I proceed ?

I thought using some concatenation (.) but I think that this solution is dirty.

Thinking in C (because I did it in C some time ago, but I want this to be done in Perl), Once I got my "`u_char packet[]`", I just had to do a `memcpy` of my user-supplied ethernet address to the 6 first bytes of my `packet[]` and it worked.

Answer: 
The [`vec`](http://perldoc.perl.org/functions/vec.html) function is used to work with a scalar value as a bit vector.

```
use strict;
use warnings;

my $packet;
# set packet to something...

my $address = '00:11:22:33:44:55';
my @bytes   = map { hex } split(/:/, $address);
for my $i (0 .. $#bytes) {
    vec($packet, $i, 8) = $bytes[$i];
}

```
```
tcprewrite --enet-dmac=00:07:0d:0b:98:00 -i orignalmac.pcap -o newmac.pcap

```