The OP wants to use a special Perl DWIM feature called "autoquoting",
meaning that in a "('key', value)" pair with "fat-comma" '=>' the
quotes are optional on the LHS : "(key => value)".

But if you put a variable or function on the LHS it will be evaluated
to the key:

$key="k"; %h=($key => 1)

And IMHO the parsing rules are not too complicated: "check for
following =>".

At least cperl-mode gets it right.

Now how do other languages solve this problem?

AFAI can see Python has nothing like autoquoting", everything unquoted
is evaluated:

x={"a":1}

And JS has autoquoting, but the LHS is _never_ evaluated:

x={a:1}

You can't make an omelette without breaking eggs.

If you want python's semantics you need python's restrictions and put
quotes!

IIRC the perldoc cites "problems with emacs" for changing this
behaviour. :)

Cheers
  Rolf