How can I extract just the unique elements of an array? is very helpful if that's what I want to do. But I need a list of all the elements that are *not* unique. And I need each element in this output list to be, itself, unique, even if it occurs more than twice in the input list. And to make matters worse, I need to do this in a for loop where I do other operations on the input list, so that shifting off one element after another is not an option. What I have at present is

my @in = qw/
    test
    foo
    test
    bar
    baz
    foo
    test
/;

my %multiples;
my %out;

for (@in) {
    if ($multiples{$_} == 1) {
        $out{$_} = 1;
    }
    $multiples{$_} = 1;
}

print "$_\n" for keys %out;
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This does what I want, but seems clunky. Is there a cute idiom for this operation?

§ George Sherston