grep checks against %seen and, the first time, sees that it's zero (false), so doesn't pass it through. The second time, it's non-zero (true), so it passes that value through. The third and subsequent times, it's still true. Your grep would have dropped all unique values. (If you had a D123, for instance, it would have been missed.)

If you absolutely have to have it in one line, provide a function called unique() and do something like:

sub unique { my %x; @x{@_} = @_; values %x }

my @sorted =
    sort {
        $a cmp $b
    } unique map {
        substr($_, 0, 1 )
    } @original;
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