$ cat 887880.pl
#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $t1 = {a=>1, b=>2, c=>3};
my $t2 = {a=>2, b=>5, c=>7};

my %H = ( X=>$t1, Y=>$t1, Z=>$t2 );

$H{Y}{b}=11;

print Dumper(\%H);
$ perl 887880.pl
$VAR1 = {
          'Z' => {
                   'c' => 7,
                   'a' => 2,
                   'b' => 5
                 },
          'X' => {
                   'c' => 3,
                   'a' => 1,
                   'b' => 11
                 },
          'Y' => $VAR1->{'X'}
        };
$