curly braces have more purposes

• grouping blocks like if (1) { CODE }
• literal anonymous hashes $href = { KEY => VALUE }
• accessing hash keys $href->{KEY} or $hash{KEY}

This my %myhash = %{ $href } ; is just a form of grouping like in the first example to clarify the precedence.

Which doesn't make much sense here since my %myhash = %$href ; has the exact same effect.

And it's not the curly which does the dereference, but the %sigil.