The (&@) prototyping mechanism is intended allow user-defined functions to enjoy an approximation of some of the behavior of built-in functions like grep and map, but does not allow exact duplication of all such behavior. I've never tried it myself, but I imagine that if you wanted to do this trick, you'd have to look at the source of the built-in you wanted to emulate and figure out how it achieves its Deep Magick, and then roll your own with, e.g., an Inline::C definition that duplicates the incantations.
Perhaps some much more subtle monk than I can comment further.
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