My query
$length = unpack "C4", substr $data,0,2,'';

Your query
my ($n) = unpack 'x v', $raw;
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You didn't show what your $data contains, but you seem to be working with the hex representation of the data instead of working on the data represented. Don't convert your data to hex; that just complicates things.

The example code in my first reply was based on certain assumptions, in particular, that the data you are dealing with is raw binary. My example code may have no bearing on the actual problem you face.

$length = unpack "C4", substr $data,0,2,'';
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As to your code, my interpretation is:

• Two characters are taken from the beginning of the string  $data and replaced with the empty string;
• An attempt is made to unpack four unsigned bytes from the extracted two-character string;
• Then the first of the four unpacked bytes (only two of which can actualy exist) is stored in  $length and the rest are thrown away.
This doesn't really seem to make a lot of sense. See code example below.

>perl -wMstrict -le
"my $data = 'ABCDEF';
 my $len = unpack 'C4', substr $data, 0, 2, '';
 print $len;
 print qq{'$data'};
"
65
'CDEF'
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