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my @hash_keys = qw/foo bar baz quux w x y z/;
my %foo;
my %bar;
...
    $bar{$key} = 1;
    print "$key: ", join(':', keys %bar), "\n";
}

foo: foo
bar: bar:foo
baz: bar:baz:foo
...
baz: w:y:baz:quux:x:z
bar: w:y:bar:baz:quux:x:z
foo: w:baz:x:y:bar:quux:foo:z