In the call to the subroutine, you're passing a (nonexistent) scalar, not the hash you think you're passing:

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

As well, in the subroutine, you're assigning a scalar value, when you want a list context:

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

These kinds of things are easy to miss, which is why it's recommended to use the strict pragma...It should kick up warning messages in such cases.

 

Addendum: Also wanted to add that if you're only ever passing a single hash to your subroutine, and it's not going to be an extremely large hash, then passing by value is workable. If you want to pass other parameters, or a massive hash (megabytes worth of data, yes I've seen this done), then you'd want to pass a reference:

# 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};
[download]