parser = new Parse::RecDescent (q{
        expression: and_expr '||' expression | and_expr
        and_expr:   not_expr '&&' and_expr   | not_expr
        not_expr:   '!' brack_expr           | brack_expr
        brack_expr: '(' expression ')'       | identifier
        identifier: /[a-z]+/i
        });

 # becomes after dropping the quoting operator q{}

    parser = new Parse::RecDescent(
        expression: and_expr '||' expression | and_expr
        and_expr:   not_expr '&&' and_expr   | not_expr
        not_expr:   '!' brack_expr           | brack_expr
        brack_expr: '(' expression ')'       | identifier
        identifier: /[a-z]+/i
        );