No reason I'm aware of; just a matter of personal preference. I tend to use a hash variable rather than a hashref at the top level to save typing arrows ( $data{level1}{level2} instead of $data->{level1}{level2} ) unless it's a reference to an object or I'm going to be passing it as a reference to a lot of functions or something, in which case I'll save a little typing by passing $data instead of \%data. So it really just depends on how I'll be using it.