Well if you've got more memory than sense, then this might suffice for your task at hand -

open(BF, $tsv_data) or die("It ain't there ($tsv_data): $!\n");
@res = map { "$_\n" if /LONDON/ } split(/\n/, <BF>);
close(BF);
[download]

Grab everyline with 'LONDON' and stick on that ever useful newline. Not necessarily a 'better' way but certainly a quicker one ;o)
HTH

broquaint