my $key3 = $hash{$key1}{$key2};

##</code><code>##

my $value = $hash{$key1}{$key2}{$key3};

##</code><code>##

%hash = ( key1 => { key2 => $some_key } );
%hash = ( key1 => { key2 => { $some_key => $some_value } } );