Strictly speaking he doesn't want the intersection. He wants to put the elements from @array_2 into a bag (Set::Bag) but only if they're present in @array_1.

my %wanted = map { $_, 1 } @array_1;
my $bag = Set::Bag->new( );
foreach( @array_2 ) {
  $bag->insert( $_ => 1 ) if $wanted{ $_ };
}
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