It's basically creating a temporary hash and testing if $field exists in the hash. So

map { $_ => undef } qw(Name Phone)
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Create a list of key value pairs from the given list where the keys are each element of the list and values are undef

{ map { $_ => undef } qw(Name Phone) }
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Populate an anonymous hash with the list that was created from the map

exists { map { $_ => undef } qw(Name Phone) } -> { $field };
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Check for the existence of $field in the anonymous hash reference.

Perl Idioms Explained - keys %{{map{$_=>1}@list}} may also be of help.
HTH

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broquaint