An an alternative:

{
   use Scope::Guard;
   my $_cache;
   sub withLongComputation {
      $_cache //= ...;
      ...;
   }
   sub localClearCache {
      my $old = $_cache;
      undef $_cache;
      return guard { $_cache = $old };
   }
}

...;

withLongComputation();
withLongComputation();

{
   my $guard = localClearCache();
   # here, $_cache has been cleared
   
   withLongComputation();
}

# Because $guard has fallen out of scope,
# $_cache has been restored to its old value

withLongComputation();
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