Here's a guess. 

my @lowercase_b = map lc, @b;
my %lowercase_a = map { lc, $_ } @a;
my @intersection_and_undefs = @lowercase_a{@lowercase_b};
my @intersection = grep defined, @intersection_and_undefs;

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"Why 1st map is enclosed by @{}" => It's a hash slice.

In reply to Re: Strange grep construct by tybalt89
in thread Strange grep construct by mzvk

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