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my @intersection = grep { defined } @{ { map { lc ,=> $_ } @a } } { map { lc } @b };
$ perl -MO=Deparse -e 'my @intersection = grep { defined } ... ' my(@intersection) = grep({defined $_;} @{+{map({lc $_, $_;} @a)};}{map + {lc $_;} @b}); -e syntax OK