my %A = (a => 1, b => 2, D => 3);
my %Z = (z => 9, y => 8, D => 7);
(\ my %all_keys)->@{keys %A, keys %Z} = ();
say for keys %all_keys;

##</code><code>##

@{ \ my %all_keys }{keys %A, keys %Z} = ();
say for keys %all_keys;  # Global symbol "%all_keys" requires explicit package name (did you forget to declare "my %all_keys"?)