I started with something like almut's state machine but didn't like the way the structure of the file I was trying to read is then encoded into the script structure - feels like poor code/data separation.

What I've ended up with is more like BrowserUk's suggestions. The crucial point (which I didn't really realise when I originally posted) was that the file format defines how many words should be read for each "variable" in the file, although in most cases that length is actually defined by algebra involving previously-read variables. So we can do something like this:

my %v; # this hash holds all the data read

# define the names of the "variables" to read from the file
# *** in the correct order ***
my @varnames = qw/this that theother/;

# define where the length to read is not a default
my %varlengths = ('that' => 2, 'theother' = '$v{this}*2'}

foreach my $var (@varnames){

 # get the length to read
 my $readlength;
 if(! exists($varlengths{$var})
  $readlength = 1; # default
 } else {
  $readlength = eval($varlengths{$var}); # compute it
 }
 
 # read into %v
 if ($readlength == 1){
  # read a scalar
  $v{$var} = unpack TEMPL1, read( $file, $readlength );
 } else {
  # read into an anon. array
  $v{$var} = [ unpack TEMPL1, read( $file, $readlength ) ];
 }

}
[download]

I think would prefer to use subs rather than string eval here. Eg.

my %varlengths = (
    'that' => sub(){ 2 }, 
    'theother' = sub{ $v{this}*2 },
}
...
  $readlength = $varlengths{$var}->(); # compute it
[download]

Slightly more work, but worth it I think.

---

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"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

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Interesting, I didn't even know you could do that! Presumably the advantage is speed? Being a compile-time thing rather than a run-time eval?

If/when I finally make progress on this I'll compare the approaches.