I thought that was weird too, so I looked it up. It turns out that, unlike most pragmas, vars is not block scoped.

Here's the snippet:

       Unlike pragmas that affect the $^H hints variable, the use
       vars and use subs declarations are not BLOCK-scoped.  They
       are thus effective for the entire file in which they
       appear.  You may not rescind such declarations with no
       vars or no subs.
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