(?:\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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