gistfile1.pl

grammar MyInteger {
	token TOP {
		| 0b(<[01]>+) {*}  #= binary
		| \d+         {*}  #= decimal
	}
}

class Twice {
	multi method TOP($/, $tag) {
		my $text = ~$/;

		# Don't do :2($text) since :2('0b1') == 23.
		# I'm not sure it's a feature or not.
		$text = :2(~$/[0]) if $tag eq 'binary';
		make $text * 2;
	}

	# This multi method will never be called
	# since every rule has ``#= tag''.
	#
	# multi method TOP($/) { }
}

say MyInteger.parse('21',      :action(Twice.new)).ast;  # 42
say MyInteger.parse('0b10101', :action(Twice.new)).ast;  # 42