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";


##</code><code>##

> perl hashofhash.pl
"my" variable %newfile_attachments masks earlier declaration in same scope 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.