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

##</code><code>##

return &Y::module_subroutine($hash_to_send);

sub module_subroutine {
   my($hash) = @_;
   return "The colour of the sky is ".
          $hash{third_colour};
}

##</code><code>##

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

sub module_subroutine {
   my(%hash) = @_;                            <--- Use the right type
   return "The colour of the sky is ".
          $hash{third_colour};
}

##</code><code>##

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

sub module_subroutine {
   my($hash) = @_;                             <--- A ref to a hash
   return "The colour of the sky is ".
          $hash->{third_colour};               <--- Must dereference
}