>Also, is there a way of creating a hash for consumption by 'keys' in the above situation which doesn't involve creating a hash reference from a list and then immediately dereferencing?

this is because hash hope first argument is a hash or array, so if offer a list the first argument is first element of list.
so keys just can't work on 'a' (in your example)...
if want keys work on return values of f(), the list still need stored into hash or array.

my %hash = f();
my @keys = keys %hash;
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but you hope finish in one line but don't want %{{}}
I can't offer any to avoid hash reference, but if the pupose just look tidy
maybe let subroutine f() return reference?
so we can use:

my @keys = keys %{f()};

#or in postfix dereferencing
my @keys = keys f()->%*;
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hope this can help.

edit:I see you say you don't want :(

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