my @top;

while (<DATA>) {
    my ($id, $score) = split ' ';
    my $e = $top[-1];  # last/lowest entry in @top
    if ( !defined($e) || $score > $e->[1] ) {
        push @top, [$id, $score];
        @top = sort {$b->[1] <=> $a->[1]} @top;
        pop @top if @top > 20;
    }
}
for (@top) {
    say "$_->[0] : $_->[1]";
}

__DATA__
ID1 25
ID2 8
ID3 3
... 42

##</code><code>##

my @top;

while (<DATA>) {
    my ($id, $score) = split ' ';
    push @top, [$id, $score];
    @top = sort {$b->[1] <=> $a->[1]} @top;
    pop @top if @top > 20;
}