why not parallel seq scan for slow functions

If I have a slow function which is evaluated in a simple seq scan, I do not
get parallel execution, even though it would be massively useful. Unless
force_parallel_mode=ON, then I get a dummy parallel plan with one worker.

explain select aid,slow(abalance) from pgbench_accounts;

CREATE OR REPLACE FUNCTION slow(integer)
RETURNS integer
LANGUAGE plperl
IMMUTABLE PARALLEL SAFE STRICT COST 10000000
AS $function$
my $thing=$_[0];
foreach (1..1_000_000) {
$thing = sqrt($thing);
$thing *= $thing;
};
return ($thing+0);
$function$;

The partial path is getting added to the list of paths, it is just not
getting chosen, even if parallel_*_cost are set to zero. Why not?

If I do an aggregate, then it does use parallel workers:

explain select sum(slow(abalance)) from pgbench_accounts;

It doesn't use as many as I would like, because there is a limit based on
the logarithm of the table size (I'm using -s 10 and get 3 parallel
processes) , but at least I know how to start looking into that.

Also, how do you debug stuff like this? Are there some gdb tricks to make
this easier to introspect into the plans?

Cheers,

Jeff