It's not clear to me what you want, since your code is wrong.

Here the correct analogous:

$ref->{foo}{$bar}

(NB: autoquoting of key "foo" !)

is

demo in the JS console

>> ref={foo: {x:42}}
Object { foo: {…} }

>> bar="x"
"x"
>> ref.foo[bar]
42
>> ref['foo'][bar]
42
[download]

as a side note, $ is a legal identifier in JS so you can also write

>> $ref={foo: {x:42}}
Object { foo: {…} }

>> $ref['foo'][$bar]
42
[download]

update

demo in the Perl debugger

  DB<3> $ref = {foo=> { x => 42 } }

  DB<4> $bar = "x"

  DB<5> p $ref->{foo}{$bar}
42
  DB<6> p $$ref{foo}{$bar}
42
[download]

update

  DB<11> use experimental 'refaliasing'; \%ref = $ref

  DB<12> p $ref{foo}{$bar}
42
[download]