Just like \@ requires that the arg starts with "@", \$ requires that the arg starts with "$".

foo(${\ "constant" });
foo(${\ bar()      });

&foo(\ "constant" );
&foo(\ bar()      );
[download]

Perl could be changed to accept constants. (I think it's based on opcodes, not symbols.) However, it defies the expectation of a writable value (\$) implies.