But what's with the "panic: top_env" message? And why doesn't it print out a diagnostic too?

This panic: message doesn't actually seem to be implemented as a warn or die; it just does the equivalent of print STDERR "panic: top_env\n"; and then bails out of the parser with an error. Perhaps this is also a bug :)

You can still get the diagnostics by piping STDERR to the splain utility:

$ 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 tha
+t
    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 t
+hat.
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