Do I misunderstand, or is the colliding file handle actually Foo::FH in the example? In this example, I would say that the problem is not the use of a global file handle, but the main script placing its code into package Foo and calling frobnicate incorrectly. The use of subroutine prototypes would either make the bug in frobnicate obvious or raise a compile-time error at line 18 when it is called with too many arguments.

I argue that lexical file handles are a neutral matter of style when they are declared at top-level (which is normally limited to the main script because modules typically provide subs but do not execute code upon loading). The real problem in the contrived example is calling a subroutine with the wrong number of arguments.

In a case where the file handle is intended to be an "environment parameter" to a subroutine, global file handles are the only option, but please do not actually do that in production code, or at least very clearly document routines that expect certain global file handles to be set up by their callers.