(the package variable (%items above) is not available).
It is not, because %items is declared as a my variable and thus available only in the the package itself, not in the main package.
If you have to access it from main, you either have to fully qualify it as %items::items, or provide methods to access it (exporting it, providing accessors or a method which returns a reference to it, etc), but it's hard to say what's best without knowing the whole scenario.
perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'