my %newhash;
@newhash{@selection} = @oldhash{@selection};
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In your example:

my %london_location;
@london_location{@location_fields} = @london{ @location_fields };
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Which is concise, but still repeats both the new hash and the selection.

FWIW in Perl 6 the following is supposed to work (but doesn't yet in Rakudo):

my %newhash = %oldhash{@selection}:pairs
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(Update: while re-reading the specification it seem it must be :p instead of :pairs).