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my %data;

...
  my ($letters, $numbers) = $k =~ /(\D+)(\d+)/;
  $data{$numbers}{$letter} = $form{$k};
}

# UPDATE (fixed indices!)
my @sorted_keys =
...
  sort { $a->[2] <=> $b->[2] or $a->[1] cmp $b->[1] }
  map { [$_, /(\D+)(\d+)/] }
  keys %form;