Here's an implementation of "Keep an array of the current top 20 ..." (as suggested by GrandFather): 

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

[download]

(where the __DATA__ is just to indicate the format — you'd of course read from your huge file instead)

Update: actually, this wouldn't work ;( — e.g. if you encounter the largest value first   So, here's a corrected version (unfortunately less performant, because @top now is sorted for every iteration): 

my @top;

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

[download]

(A quick test shows that on my somewhat aged hardware, this takes 280s for a 30_000_000 line file with random 8-digit scores — as opposed to 52s for the previous only-sometimes-correct version (40s of which are for mere reading/splitting).   This is still significantly faster, though, than reading the entire data into a hash and sorting that hash. I couldn't test the full 3e+07 lines (as I only have 4G of physical RAM), but a 3e+06 entry hash already took 220s (and 805M memory), and sorting doesn't scale linearly with size.)

In reply to Re: Dealing with Hash Table by Eliya
in thread Dealing with Hash Table by &#350;uRvīv&#337;r

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