map { $_ => undef } qw(city state country) creates the list (city => undef, state => undef, country => undef). This is an idiomatic way to initialize a hash from a list of values (used as keys).

Anyway, the code:

next if exists
       { map {$_ => undef } qw(city state country) } -> { $field };
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is redundant and too 1337 for its own good. I would write:

next if grep {$_ eq $field} qw(city state country);
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Update: oops, my mind was captured by the other grep :)