With 5.14, one can use keys, values and each directly on an unblessed hash (or array) reference or an expression that evaluates as such (although this is flagged as "highly experimental"). I don't have 5.14 and so cannot provide a working example, but I think it should go something like this:

for my $k (keys function_returning_hash_ref()) {
    print "the hash has a key $k \n";
    }

sub function_returning_hash_ref {
    return { fee => 'Fie',  foe => 'Fum', };
    }
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Update: Example tested OK with 5.14 per chromatic (with thanks!).