should it be a regex?

you can use a hash to hold the data and check if an element has occured more than once.

my %temp;
my @captured = grep { defined $temp{$_}++ } split / /, <DATA>;
[download]

If you need exactly two, try this

my %temp;
my @captured = grep { ++$temp{$_} == 2 } split / /, <DATA>;
[download]

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