That worked. Many thanks. Now I want to expand this a bit more by adding more values inside the hash:

my %hash = (
             '103496-1'  => [{ 'CLVD' => '5678', 'COMP' => '1234', 'FD
+' => '0010', 'Files' => [{'File' => 'text.txt', 'hash' => 'a538346ad3
+485'},{'File' => 'text2.txt', 'hash' => '237d97892376a'}] }]
           );

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

my %newhash;
my $fn;

foreach $fn (values %hash) {
        $newhash{$fn->{COMP}}= $fn;
        delete $fn->{COMP};
}


print Dumper(%newhash);
print "\n";
[download]

this gives me the error and wrong output as:

Pseudo-hashes are deprecated at hashofhash.pl line 18.
Use of uninitialized value in hash element at hashofhash.pl line 18.
Pseudo-hashes are deprecated at hashofhash.pl line 19.
$VAR1 = '';
$VAR2 = [
          {
            'FD' => '0010',
            'COMP' => 1234,
            'CLVD' => '5678',
            'Files' => [
                         {
                           'hash' => 'a538346ad3485',
                           'File' => 'text.txt'
                         },
                         {
                           'hash' => '237d97892376a',
                           'File' => 'text2.txt'
                         }
                       ]
          }
        ];
[download]

Any ideas?