my $tsv_data;
# Slurp the file
{
    local $/ = undef;
    local *BIGFILE;
    open BIGFILE, "<$tsv_data" or die "can't open big file. $!";
    $tsv_data = <BIGFILE>;
    close BIGFILE or die "Can't close big file: $!";
}

# Throw it all in an array for testing
my @records = split /\n/, $tsv_data;

# Opps need the line feeds back again for the output files
@records = map { "$_\n" } @records;

my @london = grep /LONDON/, @records;

# Write to new file, amend old file..
...