$ perl -Mstrict -Mwarnings -E '
    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 "@{[sort keys %all_keys]}";
'
D a b y z