They're both basically the same construct...

How do you figure? One of them is a literal identifier and the other is an expression.

... but the syntax on line #2 is a symbolic reference...

Yes, because it's not a literal identifier. Compare:

my ${      hello_world } = "hello world";

... to:

my ${ $_ = hello_world } = "hello world";

Perl complains about the latter because you can't use a scalar dereference as the name of a lexical variable in a declaration. (You don't get to use symbolic references with lexicals anyway, at least without XS.) A quick of the grammar allows you to use a block immediately following the scalar sigil, but you get the runtime error in the my op if you have anything other than a literal scalar name here.