my $N = 20;
my $collect = 5 * $N;
my $threshold;
while (<>) {
    my ($id, $score) = split ' ';
    push @top, [$id, $score] if !defined $threshold or $score >= $threshold;
    if(@top > $collect) {
        @top = sort { $b->[1] <=> $a->[1] } @top;
        splice @top, $N;
        $threshold = $top[-1][1];
    }
}
if(@top > $N) {
    @top = sort { $b->[1] <=> $a->[1] } @top;
    splice @top, $N;
}