Why does that code block (as rhesa puts it) "get closed over?"

Because the current code implementing both (?{}) and (??{}) is a hack. The code is compiled once for performance reasons. Unfortunately its not simple to make it not a closure without a performance penalty. The issue you have think about is that the code could execute in almost any context due to qr//, thus the variable binding needs to occur at match start and needs to handle the case that there are no variables with the appropriate names to bind to, etc, etc.

You can see other warts in the implementation by doing certain forms of syntax error in the code, the error message will be distinctly unhelpful, and again apparently its a real bitch to fix.