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

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

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

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