$self apparently contains the string 'Compress::Zlib::' when $self->{gz_package} attempts to use it as a hash reference. I can't find that code, so I don't know how you got into that situation, but it sounds like an instance method was called as a static method.

Update: Shoot, how did I miss that %$package, what is actually giving the error.

It looks like it's using a funky trick to check if a module was loaded — why isn't it checking %INC? — and that trick requires the disabling of a safety feature. Specifically, no strict 'refs'; must be in effect for those statements.