@U = keys ('A'=>1,'B'=>2,'Z'=>26);

That's the same as getting every second item (starting with the first) and removing duplicates. There's no good shorthand for that, even if you omit the requirement to remove duplicates.

@U = keys %{{ map { $_=>1} @NU }};

If the contents of @NU has no duplicates, then that's the same as

my @U = @NU;
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If your question is how to remove duplicates, then there's an answer in perlfaq4. Some solutions

my @U = uniq @NU;        # From List::Util
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I use the idiomatic

my %seen;
my @U = grep !$seen{$_}++, @NU;
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