update: :) good night

1. I understand what C/xa is doing, but what does @ do in this case? but what does @ do in this case?

Looks like a typo to me :)

http://perldoc.perl.org/perlpacktut.html#The-Alignment-Pit gives the use-case for this one, like a lot of these, its weird :)

Here is what I think I kinda know, remove the @ and you get

'x' outside of string in unpack
[download]

So I think @ means start from 0 , where 0 is the beginning for the innermost group (), instead of outside the end of the string :)

So C/x says read one byte/octet, then SKIP(x) that amount of bytes, so it eats the first \003, skips next 3 bytes , which are \003\003a

And its a group, so repeats that three times, each time the C/ eating up one byte, so it inches forward

Yeah, I don't think you can do this in one step, I think it's a 2 step operation like shown in the alignment pit

2. How would I make the template skip from the absolute start of the string, rather than from a relative position? e.g.:

Wait a minute, doesn't this mean you understand 1.?

I think you can't do that, now the original makes even less sense to me

Ok, here's the 2step

use 5.12.0;
my $raw = "\003\003\003abcdef";
my @offsets = unpack '(C)3', $raw;
my $chucks = join ' ', map { '@' . $_ . 'a' }  @offsets;
say $chucks;
say for unpack $chucks, $raw;
use 5.12.0;
my $raw = "\003\003\003abcdef";
my @offsets = unpack '(C)3', $raw;
my $chucks = join ' ', map { '@' . $_ . 'a' }  @offsets;
say $chucks;
say for unpack $chucks, $raw;
__END__
@3a @3a @3a
a
a
a
[download]

update: HAHAHAHAHAHAAH I DID IT  (C/x! a@)3 gets you aaa, not sure how that works, makes no sense to me, but its straight from gappy alignment pit portion

This is what I don't like about pack/unpack,