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%var       - Hash
$var{$key} - Hash element
$var       - Scalar. No relation to %var

return &Y::module_subroutine($hash_to_send);

...
   return "The colour of the sky is ".
          $hash{third_colour};
}

return &Y::module_subroutine(%hash_to_send);  <--- Use the right var

...
   return "The colour of the sky is ".
          $hash{third_colour};
}

return &Y::module_subroutine(\%hash_to_send);  <--- Pass reference

...
   return "The colour of the sky is ".
          $hash->{third_colour};               <--- Must dereference
}