From language/create-cli

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The ARGS-TO-CAPTURE subroutine should accept two parameters: a Callable representing the MAIN unit to be executed (so it can be introspected if necessary) and an array with the arguments from the command line. It should return a capture object that will be used to dispatch the MAIN unit. The following is a very contrived example that will create a Capture depending on some keyword that was entered (which can be handy during testing of a command line interface of a script):

sub ARGS-TO-CAPTURE(&main, @args --> Capture) {
    # if we only specified "frobnicate" as an argument 
    @args == 1 && @args[0] eq 'frobnicate'
      # then dispatch as MAIN("foo","bar",verbose => 2) 
      ?? Capture.new( list => <foo bar>, hash => { verbose => 2 } )
      # otherwise, use default processing of args 
      !! &*ARGS-TO-CAPTURE(&main, @args)
}

Note that the dynamic variable &*ARGS-TO-CAPTURE is available to perform the default command line arguments to Capture processing so you don't have to reinvent the whole wheel if you don't want to.