The shape of the data in the file is of almost no interest at all in determining the data structure you need. That is dictated by how you want to use the data. When you know how you want to use the data (and therefore have some idea of the data structure required) then you can figure out how to transform the file representation of the data into the internal data structure.

First, I will traverse the sorted array of parent lines. This could be derived from the keys of a hash, or from an array.

When a parent has children, I'll need to process/aggregate each of those elements, and associate the result with the parent.

I'm leaning towards this structure:

$a->[n]->[0] = arrayref to split parent lines
$a->[n]->[1] = arrayref to arrayref of split children lines
[download]

Where do you want *them* to go today?

You could load an AoA from your data file like this, which will retain the original ordering (if thats important), and allow you to sort the data pretty much any way you need to. Quite what you mean by

Data will be sorted on the d(n) field

is far from clear to me? Neither is why you think a "hashref" is relevant?

#! perl -slw
use strict;

my @data;
while( <DATA> ) {
    if( /^\s/ ) {
        push @{ $data[ -1 ] }, split;
    }
    else {
        push @data, [ split ];
    }
}

print "@$_" for @data;

=output
__DATA__
a1   b1   c1   d1   e1   f1
a2   b2   c2   d2   e2   f2
a3   b3   c3   d3   e3   f3
   p3   q3   r3
   s3   t3   u3
a4   b4   c4   d4   e4   f4
a5   b5   c5   d5   e5   f5
   p5   q5   r5
   s5   t5   u5
a6   b6   c6   d6   e6   f6
[download]

Output

C:\test>junk4
a1 b1 c1 d1 e1 f1
a2 b2 c2 d2 e2 f2
a3 b3 c3 d3 e3 f3 p3 q3 r3 s3 t3 u3
a4 b4 c4 d4 e4 f4
a5 b5 c5 d5 e5 f5 p5 q5 r5 s5 t5 u5
a6 b6 c6 d6 e6 f6
[download]

• As for the d(n) field, let's say this represents a date on each parent line. I want to sort all of the parent lines by date.
• If lines are not immediately split, then perhaps the entire line could be used as a hash key?
  
  Sorry for not explaining this better...
  

  BTW,

  You have concatenated the children lines to the parent line -- this is not what I'm trying to do.
  
  The children lines should be a sub-array of their parent.
  
  
  Where do you want *them* to go today?

• If lines are not immediately split, then perhaps the entire line could be used as a hash key?

  If you were to use the whole string as the key to the hash, what would it buy you?

  It would make for problems in building the data structure because for the compound lines, you wouldn't have the entire key when you read the first line. You'd either have to employ some readahead, or delete and re-store compound lines under new keys each time you found an extension line.

• As for the d(n) field, let's say this represents a date on each parent line. I want to sort all of the parent lines by date.

  It would be possible to sort the data prior to spliting it, but if the fields are complex (like dates) then it's much easier to do the sort after the split.

Without making any attempt to be efficient, sorting by field n (a more normal term for your d(n) nomenclature), can be very simple. This sorts the data by the (additional) last character of the 4th field:

#! perl -slw
use strict;

my @data;
while( <DATA> ) {
    if( /^\s/ ) {
        push @{ $data[ -1 ] }, split;
    }
    else {
        push @data, [ split ];
    }
}

print "@$_" for sort{
    substr( $a->[ 3 ], -1 ) cmp substr( $b->[ 3 ], -1 )
}@data;

__DATA__
a1q   b1w   c1e   d1r   e1t   f1y
a2u   b2i   c2o   d2p   e2a   f2s
a3d   b3f   c3g   d3h   e3j   f3k
   p3   q3   r3
   s3   t3   u3
a4l   b4z   c4x   d4c   e4v   f4b
a5n   b5m   c5q   d5w   e5e   f5r
   p5   q5   r5
   s5   t5   u5
a6t   b6y   c6u   d6i   e6o   f6p
[download]

Sorting by a date field is slightly more complex, but not much. I'd give an example, but as you've given dummy data, I'd have to make up the dates and I've no idea what format your data is in.

If you posted an example of your real data and explained what you are actually trying to achieve, rather than all this abstract stuff, you'd doubtless get much better answers.

---

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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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