I think you're asking about a hash of hashes? In that case, here's one way:

my %hash1 = ( foo => 'bar' );
my %hash2 = ( quz => 'baz' );
$hash1{somekey} = \%hash2;

use Data::Dumper;
print Dumper(\%hash1);
print Dumper( $hash1{somekey}{quz} );
__END__
$VAR1 = {
          'foo' => 'bar',
          'somekey' => {
                         'quz' => 'baz'
                       }
        };
$VAR1 = 'baz';
[download]

See also perlreftut and perlref.

Update: On re-reading your question, are you just asking for how to copy %hash2 into %hash1? Below, in "Case 1", the contents of %hash1 are preserved, but if there are duplicate keys in %hash1 and %hash2 then the values from %hash2 will replace those from %hash1. In "Case 2", the contents of %hash1 are simply replaced by %hash2. There are other ways to do this depending on what your needs are.

use Data::Dump;

my %hash1 = ( foo => 'bar' );
my %hash2 = ( quz => 'baz' );

# Case 1
%hash1 = (%hash1, %hash2);
# OR
$hash1{$_} = $hash2{$_} for keys %hash2;
dd \%hash1;  # { foo => "bar", quz => "baz" }

# Case 2
%hash1 = %hash2;
dd \%hash1;  # { quz => "baz" }
[download]