perl6 Need help to understand more about proto regex/token/rule

{*} tells the runtime to call the correct candidate.
Rather than force you to write {{*}} for the common case of just call the correct one, the compiler allows you to shorten it to just {*}

That is the case for all proto routines like sub, method, regex, token, and rule.

In the case of the regex proto routines, only a bare {*} is allowed.
The main reason is probably because no-one has really come up with a good way to make it work sensibly in the regex sub-language.

So here is an example of a proto sub that does some things that are common to all of the candidates.

#! /usr/bin/env perl6
use v6.c;
for @*ARGS { $_ = '--stdin' when '-' }

# find out the number of bytes
proto sub MAIN (|) {
  try {
    # {*} calls the correct multi
    # then we get the number of elems from its result
    # and try to say it
    say {*}.elems #            <-------------
  }
  # if {*} returns a Failure note the error message to $*ERR
  or note $!.message;
}

#| the number of bytes on the clipboard
multi sub MAIN () {
  xclip
}

#| the number of bytes in a file
multi sub MAIN ( Str $filename ){
  $filename.IO.slurp(:!chomp,:bin)
}

#| the number of bytes from stdin
multi sub MAIN ( Bool :stdin($)! ){
  $*IN.slurp-rest(:bin)
}

sub xclip () {
  run( «xclip -o», :out )
  .out.slurp-rest( :bin, :close );
}