This is not standard operating procedure, as far as I know.

my $p = MyPackage->new(); 
$$p{Hello} = "world"; 
print $p->to_string, "\n";
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Requiring users of MyPackage objects to know that the objects are hashrefs isn't good OO (no encapsulation). And dereferencing the hashref to use it as a tied hash seems convoluted and unnatural e.g. compared to an interface like

tie my %h, MyPackage;
$h{Hello} = "world";
# use tied to call object methods
print tied(%h)->to_string, "\n"; # -> 'Hello World'
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i.e. in the example you've given there doesn't seem to be a case for going beyond the standard tied hash semantics.