Noticed the ++ in !$seen{$_}++?

perl -MData::Dumper -e '@a=(1, 2, 1, 2, 3); my @result = grep { !$seen
+{$_}++ } @a; print Dumper(\%seen);'

$VAR1 = {
          '1' => 2,
          '3' => 1,
          '2' => 2
        };
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