Just trying to explain the "why".

It does prevent you from messing up $foo (and %$foo, including $foo->{'SOMETHING_01'}), but it doesn't prevent you from messing up that to which $foo->{'SOMETHING_01'} points, if $foo->{'SOMETHING_01'} is a reference. That's why you need to do a deep copy, as shown in other replies.

If that doesn't make sense, check out this example:

use Data::Dumper;

{
   my $ref = [ 'moo' ];
   my %h1 = ( key1 => 'val1', key2 => $ref );

   print("$_ => $h1{$_}\n") foreach (keys(%h1));
   # prints:
   # key1 => val1
   # key2 => ARRAY(0x1abf040)

   print Dumper \%h1;
   # {
   #   'key1' => 'val1',
   #   'key2' => [
   #               'moo'
   #             ]
   # }


   my %h2 = %h1;  # Shallow copy.
   delete($h2{'key1'});
   $h2{'foo'} = 'bar';
   $h2{'key2'}[0] =~ s/m/b/;


   print("$_ => $h1{$_}\n") foreach (keys(%h1));
   # prints:
   # key1 => val1
   # key2 => ARRAY(0x1abf040)

   # and not:
   # foo => bar
   # key2 => ARRAY(0x1abf040)

   print Dumper \%h1;
   # {
   #   'key1' => 'val1',
   #   'key2' => [
   #               'boo'
   #             ]
   # }
}
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Note that %h1 never changes. Only @$ref changes. It's the same as in your code. You're changing that to which $foo->{'SOMETHING_01'} points while protecting the hash ref at $foo->{'SOMETHING_01'}.