my @records = <BIGFILE>;

##</code><code>##

my @records = <BIGFILE>;
my @london = grep /LONDON/, @records;
print LONDONFILE join "", @london;

##</code><code>##

while ( my $line = <BIGFILE> ) {
   /LONDON/ ? print LONDONFILE $line,"\n" : 
              print OTHERFILE  $line,"\n";
}