My problem is that I just can't see how to code that structure up.

If I understand what you are trying to do one way I would approach this would be:

my $files = {
   filename1 => {
                   field1 => { start_position => 10,
                               length => 20 
                              }, # values as appropriate
                   field2 => { 
                               start_position => 20
                               length => 5
                              }
                   | 
                   etc. 
                   | 
           },
     filename2 => {
              | and so on...
     }
}
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that way you can access the individual members such as:

   foreach my $file ( keys %$files ){
        foreach my $field ( keys %{$files->{$file}}{
        |
        do something here 
        |
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or some such similar.

My preferred approach would be object oriented in nature where I'd create a module similar to this:

package MyDataFile;

sub new {
    my $p = shift;
    my $class="MyDataFile";
    my $self = {
          fields => { },
          filename => undef
   };
   bless $self, $class;
   return $self;
}

sub filename {
   my $self = shift;
   if ( $_[0] ) {
       $self->{filename} = shift;
   }
   return $self->{filename};
}

sub addField {
   my ($self,$field_name,$start,$length)=@_;
   $self->{fields} = {
              fieldname=>$field_name,
              start=> $start,
              length=> $length
      };
}

sub getField {
     my ($self,$fieldname)=@_;
     return $self->{fields}->{$fieldname};
}

sub getAllFieldnames {
     my $self=shift;
     return keys %{$self->{fields}};
}


1;
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There are other methods I might add, and I might be tempted as well to create a module defining a structure descriping the field attributes as well, but that's way too complicated for the sake of this discussion for now.

Given the module I just gave you, you could do something like this in your code now:

use strict;
use MyDataFile;

my @files=();

   | 
    mumble...
   |
my $df = new MyDataFile;
$df->filename('foo.blah');
$df->addField('snarf',5,20);

   | 
   etc.
    |
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Hope this is food for thought.