the second call definitely shouldn't scream "TROGDOR!"

$ cat lingering-named-parameter.p6 
use v6;

class A {

    sub format_line($line is rw, :$link_maker) {
        my $result = $line;

        if defined $link_maker {
            my $link_regex = / \[\[ (<-[\]]>+) \]\] /; # /
            while $result ~~ $link_regex {
                $result .= subst( $link_regex, { $link_maker($0) } );
            }
        }

        return $result;
    }

    sub format_paragraph($paragraph, :$link_maker) {
        return map { format_line($^line, :$link_maker) },
               $paragraph.split("\n");
    }

    method format($text, :$link_maker) {
        return format_paragraph($text, :$link_maker);
    }
}

say A.new.format("a [[link]], sir", { "TROGDOR!" });
say A.new.format("a [[link]], sir") # look, no closure

$ perl6 lingering-named-parameter.p6 
a TROGDOR!, sir
a TROGDOR!, sir