Untested...

   foreach my $first ( keys %( $hash1{$key} ) ) {
      $hash2{ $first }{NextNum} = $hash1{ $key }{ $first }{NextNum};
      # etc.
   }
[download]

(I should have used $FileNumber rather than $first; sorry.)

Beware of possible loss of data if you have duplicate NextNum values in %hash1.