I would suggest not using a package variable at all
Agreed. I see in On Interfaces and APIs
that Conway's SAT talks propose some sound general interface advice namely:
Make common usage the default; allow uncommon usage via optional attributes
... with hashes being the natural way to implement that in Perl.
In your Foo::foo(bar=>2) example, I note you used a hash rather than a hash ref.
I've always used a hash ref, as recommended in PBP ...
though, as indicated here, that was an error (compile-time vs run-time boo-boo)
in the book from TheDamian - though he still stands by the advice.
As a Perl beginner, I chose a simple non-OO interface in
the Acme::EyeDrops sightly
function way back in 2001, four years before PBP was published.
I remember liking that interface at the time because it allowed me to add new attributes in the future without breaking old programs.
Plus I've always hated global variables (in any language) and couldn't bring myself to create a public global variable
in my first Perl module. :)