Are curly braces around Variable name in Perl optional?

In some perl code I saw the syntax ${varname}. Except from the case of string interpolation, i.e., when used outside of "...", and except the special case where varname would begin with a ^ (see here) I think it should be completely equivalent to $varname. This is what I guess, but could not find any reference to it. Can somebody help me with references about this question, or, if I'm wrong, explain to me what this syntax does? Many thanks.