Well... I can offer a few observations on this code:

  1: open (DATA, "$inputdir/$inputfile");
  2: while (<DATA>) {
  3:   my ($filehdr, $data) = m|(.{40})(.*)|;
  4:   ($len = substr($data, $offset, 4)) =~ s/^0//g;
  5:
  6:   foreach $val ($data) {
  7:     $cdata = substr($val, $offset, $len);
  8:     print "$cdata\n";
  9:   }
 10: }
 11: close(DATA);
[download]

• first: no use strict or use warnings... so the fact that you never give $offset a value is not pointed out to you.

• two of your variables are declared "my", but not the others... which isn't encouraging, either.

• line 1: the DATA file handle has a particular use. There is no need to use it for your input file, and doing so makes things less clear than they might be.

• also line 1: adding at least an or die "$!" after the open is recommended -- there is no point in proceeding if the file open fails, and if it does, it will be helpful to know why it fails.

• line 2: while (<DATA>) -- what do you expect this to do ? What I expect it to do is to read the file, line by line -- where $/ specifies the current line-ending. I see nothing in your description of the input file that suggests that the "records" are separated by "\n". Indeed the description suggests that it is a continuous stream of hex characters ([0-9A-F]), in which case <DATA> will read the entire file in one gulp... (assuming $/ has not been set to anything) In which case, why bother with the while ?

• line 3: this sets $filehdr to be the first 40 bytes of the current input "line", and $data to be the rest. I'd be tempted to do this with substr, being more direct and probably faster. But what you have will work.

• line 4: the $offset value is never set, and will be treated as zero. I assume that it is supposed to be the offset within $data of the next "record". If so, then the fact that it's not set to anything will be a big part of why your code only does something with the first "record".

• also line 4: it appears that the length is a decimal value. Since the rest appears to be hex, that bothers me...

• line 6: what do you expect foreach $val ($data) to do ? foreach works its way through a LIST -- the list here is exactly one element long... so not much looping involved.

• line 7: again uses $offset which has no value.

But do not despair... the solution is close. Consider: what $offset is doing; how you should update it after each "record"; and then how to recast your loop to work your way along the $data string. You could think about an initial value for $offset, which could eliminate the need to split the input into $filehdr and $data.

As it happens, I would still use unpack for this, but substr will also get the job done.

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