Thanks! I do think the * is prettier than $_. :)

According to the Whatever docs, if you call a method on * then that creates a lambda (anonymous function):

<a b c>.map: *.uc;     # same as <a b c>.map: -> $char { $char.uc }
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Let's test that out on one of the examples in my question. I'll test whether:

 Str $file where *.IO.f = 'file.dat',
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...is the same as:

Str $file where -> $x {$x.IO.f} = 'file.dat',
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Here's the full code with a file name, a.txt, that exists in the current directory:

#b.raku

sub MAIN(
  Str $file where -> $x {$x.IO.f} = 'a.txt',
  Int  :$length = 24,
  Bool :$verbose
) {
    say $length if $length.defined;
    say $file   if $file.defined;
    say 'Verbosity ', ($verbose ?? 'on' !! 'off');
}

--output:--
$ raku b.raku
24
a.txt
Verbosity off


$ raku b.raku --verbose --length=7 a.raku
7
a.raku
Verbosity on
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It looks to me like when you specify a code block on the right of where, the code block is called with the term to the left of where as the argument.

I don't think the if statements on the end of $length and $file actually do anything: with those default values both variables will be defined.