I want to copy a nested data structure. The FAQ tells me:

The Data::Dumper module on CPAN is nice for printing out data structures, and FreezeThaw for copying them. For example:

      use FreezeThaw qw(freeze thaw);
      $new = thaw freeze $old;

Where $old can be (a reference to) any kind of data structure you'd like. It will be deeply copied.

However, the following program:

use FreezeThaw qw(freeze thaw);
use Data::Dumper;

$hashref = { a => {}, b => {} };

print Dumper($hashref);
$copy = thaw freeze $hashref;
print Dumper($copy);
[download]

gives the following output:

$VAR1 = {
          'a' => {},
          'b' => {}
        };
$VAR1 = 1;

What's the problem?