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