Seconded on using a lexical to hold your filehandle, although foo( \*HANDLE ) would do in a pinch (just remember that the filehandle "slot" for a print statement needs to be a simple scalar variable; you would need to use a block (print { $var[$index] } "Blah\n") or store something more complicated in a temp scalar otherwise).

Having said that, something else in your code jumps out as meriting comment: you really don't want to be using prototypes; they're meant for making subs perform like builtins, not for declaring number-of-arguments. If you really want behavior more like what's normally referred to as prototypes in other languages look at something like Params::Validate.