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?

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