sub killperformance {
    eval q{
        use re qw(debug);
        $&;
    };
}
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The re pragma is not lexical :( This is a terrible performance killer, most of the time even worse than $&. And since the output goes to stderr, which is the error log under mod_perl, it can take a while to discover that it's still in effect globally.

Three very well known and widely used modules that use $& are Parse::RecDescent, Text::Balanced and XML::Twig.