for my $k (@sorted_keys) {
    $hash{$type}{$k}{data} = shift @mydata;
}
[download]

---

The closest that I can come to what you have written is something like this:

my @subfields = qw( REQ POS LENGTH data FILTER );
my @sorted_keys = sort qw( field1 field2 );
my @mydata = ( [ 'Y', 34, 10, ' ' x 10, \&default_filter ],
               [ 'Y', 35, 10, ' ' x 10, \&default_filter ] );
my $type = 'that_type';
for my $k (@sorted_keys) {
  @{ $hash{$type}{$k} }{ @subfields } = @{ shift @mydata };
}
[download]

which doesn't seem to me terribly convenient. An improvement would be:

use strict;
my @subfields = qw( REQ POS LENGTH data FILTER );
my @mydata = ( [ this_type =>
                 [ 'N', 34, 10, ' ' x 10, \&default_filter ] ],

               [ that_type =>
                 [ 'Y', 34, 10, ' ' x 10, \&default_filter ],
                 [ 'Y', 35, 10, ' ' x 10, \&default_filter ] ],
             );
my %hash;
for my $d ( @mydata ) {
  my $type = shift @$d;
  for my $data ( @$d ) {
    my $field;
    @{ $field }{ @subfields } = @$data;
    push @{ $hash{ $type } }, $field;
  }
}
__END__
  DB<2> x \%hash
0  HASH(0x82ea2dc)
   'that_type' => ARRAY(0x8449fa0)
      0  HASH(0x8449f10)
         'FILTER' => CODE(0x814cdd4)
            -> &CODE(0x814cdd4) in ???
         'LENGTH' => 10
         'POS' => 34
         'REQ' => 'Y'
         'data' => '          '
      1  HASH(0x8449fac)
         'FILTER' => CODE(0x814cdd4)
            -> REUSED_ADDRESS
         'LENGTH' => 10
         'POS' => 35
         'REQ' => 'Y'
         'data' => '          '
   'this_type' => ARRAY(0x8449ef8)
      0  HASH(0x8449e20)
         'FILTER' => CODE(0x814cdd4)
            -> REUSED_ADDRESS
         'LENGTH' => 10
         'POS' => 34
         'REQ' => 'N'
         'data' => '          '
[download]

As you can see, I got rid of the fieldn keys (in this regard you may want to read this node). Now the values of %hash are (references to) arrays containing (references to) hashes. E.g., to access the LENGTH subfield of the second field of that_type, you'd use

$hash{ that_type }[ 1 ]{ LENGTH }
[download]

Also note that

\&default_filter()
[download]

and

\&default_filter
[download]

mean different things. The former (which is what you used in your code), evaluates to a reference pointing to whatever the subroutine default_filter returns. In contrast, the latter evaluates to a reference to the subroutine default_filter itself. In the code I posted, I assumed that what you really wanted was the latter.