$ perl -e'qr/(?{rand < 5})/' 2>&1 |splain
Warning: Use of "rand" without parentheses is ambiguous at (re_eval 1) line 1 (#
1)
    (S ambiguous) You wrote a unary operator followed by something that
    looks like a binary operator that could also have been interpreted as a
    term or unary operator.  For instance, if you know that the rand
    function has a default argument of 1.0, and you write

        rand + 5;

    you may THINK you wrote the same thing as

        rand() + 5;

    but in actual fact, you got

        rand(+5);

    So put in parentheses to say what you really mean.

panic: top_env (#2)
    (P) The compiler attempted to do a goto, or something weird like that.