If you wish to force Perl to interpret a set of braces as an anonymous hash composer rather than a BLOCK in a situation where it otherwise wouldn't, you should use the unary (+) operator. So your code becomes:

sub make_hashref3 {
  +{
    %{ make_hashref() },
    boo => 'too',
  };
}
[download]

If this looks ugly to you, use return:

sub make_hashref3 {
  return {
    %{ make_hashref() },
    boo => 'too',
  };
}
[download]

I added semicolons to both examples for added clarity, though they are certainly optional.