The result of this code: {map {$_ => undef } qw(Name Phone)} is this structure:

{ "Name"  => undef, 
  "Phone" => undef}
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Then it is checked if $field is a key in this structure.

More precisely what I called here a structure is a pointer to an anonymous hash. Thats why the exists function can be used.

Hope that explains how it works. But I what I can't explain is why is it so obfuscated.