Oh! I wasn't clear that you controlled both. You could do a form of protected access that returns a reference to the structure. (Breaking encapsulation in a limited way).

# in MathML::Entities
my %ENTITIES; 

sub _get_entity_hash {
  my $caller = caller;
  if ( $caller->isa("MathML::Entities") ) {
    return \%ENTITIES;
  }
  else {
    die "_get_entity_hash can't be called from $caller"
  }
}
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