No, I don't know why. Curlies are always used where are variables name is expected.

my $x = ${$hash_ref}{key};  # Original syntax
my $x = $hash_ref->{key};   # Newer, cleaner syntax

print($foo, "bar\n");
print("${foo}bar\n");       # same
print("$foobar\n");         # XXX

my $x = ${"varname"};       # Symbolic reference
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etc.