(?:\z(?{ die })|)
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(?(?=\z)(?{ die }))
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It's too bad your Perl segfaults on die from inside a regex.. that doesn't happen for me (5.8.0 Linux nothreads).

Your troubles with using a lexical are possibly due to these code blocks inside regexes being closures; were you aware of that?

Unfortunately, there is currently no way to tell the regex engine to fail the entire match immediately, which is why die is necessary. It will work in Perl6, but then, so will matching on streams.. :)

The only solution is to do what we did in the days of Pascal to cope with the lack of last and friends: nest conditionals. In terms of the pattern matching, that means an attempt to match

.*?abc(def)?
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(?(?!\z)
    .*?
    (?(?!\z)
        abc
        (?(?!\z)
            (def)?
            |
            (?{ $PREMATURE_INPUT_END++ })
        )
        |
        (?{ $PREMATURE_INPUT_END++ })
    )
    |
    (?{ $PREMATURE_INPUT_END++ })
)
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Just a quick note on using lexicals in those regex closures. The trouble was that the lexicals were seen sometimes. I could not find out when they were seen I<exactly>. If those regex code constructs worked alright as closures, they would have seen the lexicals I<all the time> because the closures were declared and used I<in the same scope> that the lexical was declared. What happened was, usually the first two times the regular expression engine executed the closure, the lexical would be incremented, and after that, the code would still be executed (print() worked), but the lexicals weren't touched. (I tried it with a tied lexical that warns when touched, too.)

Steffen