Here's a sample that shows your second line actually works.

#!/usr/bin/perl

use strict;
use Data::Dumper;

my %hoh;
my %hash = ( 'a' => 1, 'b' => 2 );

$hoh{'key'} = \%hash;
print Dumper \%hoh;
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The output is:

$VAR1 = {
          'key' => {
                     'a' => 1,
                     'b' => 2
                   }
        };
[download]