The error indicates the regex compiler didn't think it had everything it needed to create the regex complete with eval contents at compile time. The first thing here is that your inclusion of $_ defers the overall regex compilation time until runtime due to interpolation. At that point its just a distinction between whether "(?{1})" is precompiled or not for (??{ ... }).

My guess is that you managed to stave off the runtime-eval error by tricking perl with the qr// operator. In reality that qr// wasn't compiled until the contents of the (??{ ... }) were being parsed/compiled at runtime. The difference is probably that the (?{1}) was considered safe by qr// because it was during *a* compiletime. That really doesn't sound like a correct behaviour to me. Bug?

My guess is that you managed to stave off the runtime-eval error by tricking perl with the qr// operator

Maybe. Although putting another eval string around it doesn't seem to change anything:

$ perl -Mre=eval -e '/ (??{ eval "qr<(?{1})>" }) $_ /x'
$ perl -Mre=eval -e '/ (??{ eval "q<(?{1})>" }) $_ /x'
Eval-group not allowed ...
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perlre has an explicit distinction for interpolating qr// objects (under (?{}) instead). The problem is that I do use re "eval", but perl seems to forget. It does seems a little bug-ish, I'll perlbug it.

perlre: (?{ })
For reasons of security, this construct is forbidden if the regular expression involves run-time interpolation of variables, unless the perilous "use re 'eval'" pragma has been used (see re), or the variables contain results of "qr//" operator (see "qr/STRING/imosx" in perlop).

And of course, the bug would be that perl isn't throwing the error but should.

Date: 27 Aug 2003 12:47:56 -0000
From: Rafael Garcia-Suarez (via RT) 
Subject: Re: [perl #23569] use re 'eval'; lost in (??{"(?{1})"})

Brad Bowman (via RT) wrote:
> This code generates an "Eval-group ..." error despite the  presense
> of "use re qw(eval)".
> 
> $ perl -Mre=eval -e '/ (??{ "(?{1})" }) $_ /x'
> Eval-group not allowed at runtime, use re 'eval' in regex m/(?{1})/ 
+at -e line 1.

This is apparently just another instance of the compiler hints not bei
+ng
propagated to eval-strings (except "integer", "strict refs" and a few
others). This can't be fixed non-kludgily without an overhaul of the h
+int
system.
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What are you trying to do with your code?

I'm just playing around.

Returning a qr// regex is a work around.

I read perlre and re, and I consider "(?{1})" a valid return value. You can use other strings without a problem:

$ id|perl -nle 'print /( (??{ q[uid=\d+] }) )/x'
uid=1000
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This snippet from perlre doesn't specify a regex ref or string (although it is pretty vague). The example which follows in the man page uses a recursive qr// definition (so does return a qr// object).

From perlre:
(??{ code })
... The result of evaluation is considered as a regular expression and matched as if it were inserted instead of this construct.

My apologies, seems as though you were right. However, I found an interesting section in perl5005delta:
%s: Eval-group not allowed at run time
(F) Perl tried to compile a regular expression containing the (?{ ... }) zero-width assertion at run time, as it would when the pattern contains interpolated values. Since that is a security risk, it is not allowed. If you insist, you may still do this by explicitly building the pattern from an interpolated string at run time and using that in an eval(). See perlre/(?{ code }).
Perl just doesn't seem to like what you're doing and stops it ;-).
Hope this helps.
Cheers, CombatSquirrel.

Update: fixed typos.