What you want to do is take each key name and split it into the letter half and the digit half, and use these as keys in a multi-dimensional hash:

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};
}
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Now your %data hash has the I, Q, P, and IT fields grouped by the number following them.

If you'd rather just sort the keys in this manner and not produce a whole new data structure, I suggest:

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