# you're doing:
return &Y::module_subroutine($hash_to_send);
# should be:
return &Y::module_subroutine(%hash_to_send);

##</code><code>##

# you're doing:
my($hash) = @_;
# should be:
my %hash = @_;

##</code><code>##

# passing it in...
return &Y::module_subroutine(\%hash_to_send);

# and assigning it within the subroutine...
my $hash = shift;

# and when using it in the sub, be sure to dereference...
return "The colour of the sky is " . $hash->{third_colour};