The other day when I was working on Perl's regexp (?{...}) construct and constraining matches. I came across a strange error message. Before I start in, let me state that I'm fully aware that (?{...}) is considered a "highly experimental" feature, and that I should expect quirks. But that said, here's the background:

The following code will produce a runtime warning:

print rand + 5;

The warning will be:

Warning: Use of "rand" without parentheses is ambiguous at mytest.pl line 4 (#1)

This is understandable (though it's one case where whitespace does matter, as "print rand +5;" doesn't trigger a warning), and the information returned from 'use diagnostics' pretty much tells the story:

Warning: Use of "rand" without parentheses is ambiguous at mytest.pl l
+ine 4 (#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.
[download]

Now look at the following snippet, which commits the same faux pax within a (?{ code }) block of a regular expression:

use strict;
use warnings;
use diagnostics;

my $string = "1230000000";

print $string =~ m/123(?{(pos <= length($_)*.5) ? '(?=)':'(?!)'})(??{$
+^R})/
    ? "Matched.\n"
    : "Didn't match.\n";

__OUTPUT__
Warning: Use of "pos" without parentheses is ambiguous at (re_eval 2) 
+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
[download]

Again, the same warning, and the same diagnostic message: no problem. But what's with the "panic:  top_env" message? And why doesn't it print out a diagnostic too?

Looking in perldiag I found the following:

(P) An internal error you should never see (trappable).
....
panic: top_env
(P) The compiler attempted to do a goto, or something weird like that.

First, what is this? Second, since I "should never see" this, does the fact that I am seeing it indicate that I should submit a perlbug report?

With similar snippets I've also managed to trigger the following similar error:

panic: restartop 
(P) Some internal routine requested a goto (or something like it), and
+ didn't supply the destination.
[download]

Though I can't remember how to reproduce the latter, they seem related. I cannot help but think that if I'm seeing these sorts of messages, there is a loose end somewhere in Perl that needs to be tied up. Any voices of agreement or disagreement?

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