• Windows:

  You can obtain the ANSI code page using GetACP. It's trivial to write Win32::API glue for that function.

  For existing files, you can use Win32::GetANSIPathName.

• Unix:

  open's ":locale" uses the following to determine the appropriate encoding:

  require encoding;
  $locale_encoding = encoding::_get_locale_encoding();
  [download]

I don't know if anyone got around to making this usable.