sub follow(IO::Handle $fh --> Supply) {
    $fh.seek(0, SeekFromEnd);
    supply {
        while True {
            my $line = $fh.get;
            with $line {
                emit $line;
            } else {
                sleep 0.1;
            }
        }
    }
}

sub grep($pattern, Supply $lines --> Supply) {
    supply {
        whenever $lines -> $line {
            if $line.contains($pattern) {
                $line.emit;
            }
        }
    }
}

sub MAIN(Str $file) {
    my $fh = open($file);
    react {
        whenever $fh ==> &follow() ==> &grep("perl6") {
            .say;
        }
    }
    $fh.close();
}