It doesn't work because there isn't anything in %hash (or %$hashref) at the time $hashref->{key1}{key1_1} is evaluated. The assignent of the list to %hash (and therefore to %$hashref) only happens when the ")" is reached. The following would work:

my %hash;
my $hashref = \%hash;
$hash{key1} = { key1_1 = ["value1_1",value1_2"],
                key1_2 = "w00t!",
              };
$hash{key2} = { key2_1 = "wakkawak!", 
                key2_2 = $hashref->{key1}{key1_1}, 
              };
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so would:

my $it = ["value1_1",value1_2"];
my %hash = ( key1 => { key1_1 = $it,
                       key1_2 = "w00t!",
                     },
             key2 => { key2_1 = "wakkawak!", 
                       key2_2 = $it, 
                     },
);
my $hashref = \%hash;
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