In 5.10, there's a much easier solution: (?|). 
    $_ = "caaat";
    my @hits = /(?|c(a)|k(t))/g;
    say scalar @hits;
    say @hits;
    __END__
    1
    a

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In reply to Re^2: empty hits with regex by JavaFan
in thread empty hits with regex by Anonymous Monk

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