my %data;

for my $k (keys %form) {
  # assumes all the keys are non-numbers followed by numbers
  my ($letters, $numbers) = $k =~ /(\D+)(\d+)/;
  $data{$numbers}{$letter} = $form{$k};
}

##</code><code>##

# UPDATE (fixed indices!)
my @sorted_keys =
  map { $_->[0] }
  sort { $a->[2] <=> $b->[2] or $a->[1] cmp $b->[1] }
  map { [$_, /(\D+)(\d+)/] }
  keys %form;