For the particular problem you're approaching, it looks like the structure of a list of pairs is quite adequate.

( Batch_Agency => { range => [ 1 .. 3] },
  Batch_Date   => { range => [ 4 .. 11] },
  Batch_Type   => { range => [ 12 .. 12 ] }
}
[download]

With the list of pairs, you get the features:

• every field has a name
• the order is preserved

So it should be quite easy with a little processing both to format the records and to map the input given as array of hash refs to the right positions and shapes.

Beware of using the ranges as you mentioned (range => [1 .. 3]). They can become a burden to maintain as fields are added or change size. It would probably be more useful to work only with lengths and compute automatically the range (if needed). Something like length => 3 or format => 'A3'.

If you decide to avoid the suffering of writing and maintaining the code for producing these streams of fixed-format records, you may try some modules from CPAN. Among them, Data::FixedFormat and AnyData::Format::Fixed.