if (grep { /^(dualvar|set_prototype)$/ } @_ ) {
    require Carp;
    Carp::croak("$1 is only avaliable with the XS version");
  }
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this code is wrong because it uses $1 not immediately after obtaining it. require Carp is probably overwriting it (it's a global var, after all, overwritten in every capturing regexp), and indeed the error message I was getting (" is only avaliable") can be explained by a failed match or an empty capture.

Nope. require Carp can't overwrite a different scope's $1. Even if (grep { /(...)/ } "abc") {print ":$1:"} has $1 undefined.

$1 is not "global", it is automatically dynamically scoped.

The problem is that the code is using $1 outside the scope in which it was set (the grep block). The code as written will never use the $1 from the match.