This suggestion certainly won't help your efficiency, but simply grepping for lines that match /LONDON/ doesn't meet your specs. How important the difference is, I don't know, but to meet those specs (and assuming $n holds the number of the field you want):

my @london = grep { (split /\t/)[$n] =~ /LONDON/ } grep { /LONDON/ } @
+records;
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This would only perform the split on the members of @records that matched somewhere.

perl -e 'print "How sweet does a rose smell? "; chomp ($n = <STDIN>); 
+$rose = "smells sweet to degree $n"; *other_name = *rose; print "$oth
+er_name\n"'
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