my @hash_keys = qw/foo bar baz quux w x y z/;
my %foo;
my %bar;

for my $key (@hash_keys)
{
    $foo{$key} = 1;
    print "$key: ", join(':', keys %foo), "\n";
}
for my $key (reverse @hash_keys)
{
    $bar{$key} = 1;
    print "$key: ", join(':', keys %bar), "\n";
}