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in reply to Explanation of mapping and undef

next if exists
       { map {$_ => undef } qw(city state country) } -> { $field };

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This is creating a temporary anonymous hash where the keys are the values in the qw// and then checking for the existence of $field in that hash. For a breakdown of a similar usage of temporary anonymous hashes see. Re: Re: Re: Unique values in array.
HTH

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broquaint

In Section Seekers of Perl Wisdom