Since nobody else mentioned it, there's a key difference between your two code snippets. In your first, you appeared to be interested in making the caller's hash empty except for one key/value pair. Of course, you see now why that attempt failed in modifying the caller's hash at all. In your second, you simply added or replaced a single key/value pair, and left any other contents of the caller's hash alone.

sub foo {
  my $ref = shift;

  %$ref = ( all => new );
}
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This alternative will keep the caller's hash intact, but replace ALL of its contents with a completely new set of contents, consisting of one key/value pair. This will also work even if the caller's hash is actually a blessed reference; the existing blessing is not broken (desecrated?) by this method.