Of course, ++merlyns post shows what an elegant solution is... So that's the cute idiom you where asking for!

Assuming you /can|want/ not process the duplicates right away, you could save memory by omitting the extra array @out. When you go over the array for the fist time, do the counting, then do the processing separately:

my @in = qw(test foo test bar baz foo test);

my %keycount;

  foreach (@in)
  {
    $keycount{$_}++;
    # do whatever else you need to do
  }
  
  foreach (keys %keycount)
  {
    #print $_ if $keycount > 1;
    print if $keycount{$_} > 1; # Thx, ++Aristotle!

    # or do something else with $_
  }
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So long,
Flexx