Thanks everyone for your quick responses. Basically here's the deal:
Thanks especially sauoq for the perl -le 'print prototype("CORE::open")' prototype
Thanks to others for other ideas for overloading the existing open, however that's not the aim. However, it was good to see how to overload such things as open. I might put an optional override in the module.
Thanks Chromatic for your suggestion of a separate routine, however I'd like to be able to just replace existing open() commands with my_open() commands and have any 'print FILEHANDLE' commands work.

Once again, thanks everyone for your very quick responses. This really is the most useful community I've ever been a part of.