#!/usr/bin/perl -w
use strict;
use Data::Dumper;
my %hash = ();
my $hashref = \%hash;
%hash = (
    key1 => {
        key1_1 => ["value1_1","value1_2"],
        key1_2 => "w00t!",
    },
    key2 => {
        key2_1 => "wakkawak!",
        key2_2 => $hashref->{key1}, # update!
    },
);

print Dumper($hashref);

##</code><code>##

#!/usr/bin/perl -w
use strict;
use Data::Dumper;
my %hash = ();
my $hashref = \%hash;
%hash = (
    key1 => { 
        key1_1 => ["value1_1","value1_2"],
        key1_2 => "w00t!",
    },
    key2 => { 
        key2_1 => "wakkawak!", 
    },
);

$hash{key2}{key2_2} =  $hashref->{key1}; #update!

print Dumper($hashref);