What does Any.match do?

.match is a search for a needle in a single haystack string. An infinite sequence stringifies to '...'.

say (1,3 ... 9).Str;        # 1 3 5 7 9
say (1,3 ... 9).match: '1'; # ｢1｣

say (1,3 ... *).Str;        # ...
say (1,3 ... *).match: '.'; # ｢.｣

How I worked this out

First, you're looking at the wrong method definition:

method match(Any:U: |) { ... }

Any:U is kinda like Any $ where not .defined except if it matched you would get the error message "Parameter '<anon>' of routine 'match' must be a type object of type 'Any', not an object instance ...".

But you're passing a defined Seq. So your .match calls don't dispatch to the method definition you're looking at.

To find out what a method dispatches to, use:

say (1,3 ... *).^lookup('match').package ; # (Cool)

A defined Seq will thus dispatch to the Cool code:

method match(Cool:D: |c) {
    ...
    self.Stringy.match(|c)
}

So, next:

say (1,3 ... *).^lookup('Stringy').package ; # (Mu)

And the code:

multi method Stringy(Mu:D $:) { self.Str }

So check:

say (1,3 ... *).Str; # ...

Bingo.

And confirm:

say (1,3 ... *).match: '.'; # ｢.｣