Please give the attached patch a try. I'll note that it's still a little fluid, but I'd like to get confirmation that it works for you before spending much more time going down this path.
Since this behavior goes against the spec and it worked for one reason or another in a previous release, I'm not planning on enabling this functionality by default. To enable support for this feature, you'll have to set a new comparability flag.
The gist of this patch is that I mapped all positional parameters to a stringified version of the param ($1=>:_1). This way the runtime thinks the user has named parameters and the user thinks they're using positional parameters.