But what you have here in your examples are not hashes, these are array references. Don't get confused by the =>, that's just a fancy way of writing comma.

Hash of Hashes or Array of Hashes? Or Hash of Arrays which is what your syntax suggests?

Lets say it is an array of hashes:

my @new;
foreach $fn (@old) {
  $new[$fn->{FirstNum}]= $fn;
  delete $fn->{FirstNum}; 
}
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If it is a hash of hashes:

my %new;
foreach $fn (keys %old) {
  $new{$fn->{FirstNum}}= $fn;
  delete $fn->{FirstNum}; 
}
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UPDATE: As an afterthought: Use Data::Dumper. It will show you the right syntax you should have used in your question. And also it will tell you which of the wildly different answers posted in this thread does what you need, if you use it to print the data structure before and after.

Here is what code I have written with a small change to the input hash:

my %file_attachments = (
             '103496-1'  => { 'CLVD' => '5678', 'COMP' => '1234', 'FD'
+ => '0010', 'File' => 'text.txt'},
                       );

print Dumper(%file_attachments);
print "\n";

my %newfile_attachments;
my $fn;

my %newfile_attachments;
foreach $fn (keys %file_attachments) {
  $newfile_attachments{$fn->{FirstNum}}= $fn;
  delete $fn->{FirstNum};
}



print Dumper(%newfile_attachments);
print "\n";
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and my output looks like:

> perl hashofhash.pl
"my" variable %newfile_attachments masks earlier declaration in same s
+cope at hashofhash.pl line 17.
$VAR1 = '103496-1';
$VAR2 = {
          'FD' => '0010',
          'COMP' => '1234',
          'CLVD' => '5678',
          'File' => 'text.txt'
        };

Can't use string ("103496-1") as a HASH ref while "strict refs" in use
+ at hashofhash.pl line 19.
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Ah yes, should have tested my code. The first error is because you defined %newfile_attachments twice. The second error goes away if you use this:

foreach $fn (values %file_attachments) {
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In your example you don't have a key 'FirstNum' in the hash. Be prepared to see an error message "Use of uninitialized value..." because of this. If you have subhashes with no key 'FirstNum', then you have to test for that and do something sensible with those subhashes

Btw, the output of Dumper looks much better if you use Dumper(\%x) instead of Dumper(%x)

This will do the job:

my %HoH = map { delete $_->{FirstNum} => $_ } @AoH;
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Be aware that you will loose data if there is duplication of "FirstNum" values.

   foreach my $first ( keys %( $hash1{$key} ) ) {
      $hash2{ $first }{NextNum} = $hash1{ $key }{ $first }{NextNum};
      # etc.
   }
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Beware of possible loss of data if you have duplicate NextNum values in %hash1.

my %hash = (
             '103496-1'  => [{ 'CLVD' => '5678', 'COMP' => '1234', 'FD
+' => '0010', 'Files' => [{'File' => 'text.txt', 'hash' => 'a538346ad3
+485'},{'File' => 'text2.txt', 'hash' => '237d97892376a'}] }]
           );
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my %newhash = (
             '103496-1'  => [{ 1234 => {'CLVD' => '5678', 'FD' => '001
+0', 'Files' => [{'File' => 'text.txt', 'hash' => 'a538346ad3485'},{'F
+ile' => 'text2.txt', 'hash' => '237d97892376a'}] }]
           );
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A question: What are you trying to do? Learn about HoHs ? Or solve a real problem? Solve several problems? Convert old data to some new format?

The main reason is that its a real problem and I need to change the format of the input to make it easier to read into another program. Basically I have no control of the original structure of the data so thats why I need to transform it into this.

This will do the job:

my %newhash = map {
                    $_ => [
                        map {
                            { delete $_->{COMP} => $_ }
                        } @{$hash{$_}}
                    ]
                } keys %hash;
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EDIT:

Or may be :

my %newhash = map {
                    $_ => [{
                        map {
                            delete $_->{COMP} => $_
                        } @{$hash{$_}}
                    }]
                } keys %hash;
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depending on what you want as final result: your example is vague because you have only one element in the inner array reference

If it was my program I will be looking for result like:

{
    '103496-1' => {
        '1234' => {
            'FD' => '0010',
            'CLVD' => '5678',
            'Files' => [
                {   'hash' => 'a538346ad3485',
                    'File' => 'text.txt'
                },
                {   'hash' => '237d97892376a',
                    'File' => 'text2.txt'                  
                }
            ]
        }
    }
};
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because you do not need an arrayref with just one element - hash reference. In which case use the following:

my %newhash = map {
                    $_ => {
                        map {
                            delete $_->{COMP} => $_
                        } @{$hash{$_}}
                    }
                } keys %hash;
[download]

Best regards

This works great and I appreciate all of your help. I'm not that familiar with the map function but it has worked for me well so far. I'm now trying to incorporate multiple hashes rather than above where I have just one and it doesn't seem to work. I've wrapped a foreach statement around the code but would that mess up the map function?