my %HASH;
foreach('AA' .. 'ZZ') {
    $HASH{$_} = 1 + int rand 1000;
}

my $keep = 20;
my @top;
if(scalar keys %HASH <= $keep) {
    # You actually need to show them all.
    @top = sort { $b->[1] <=> $a->[1] } map [$_, $HASH{$_}], keys %HASH;
} else {
    my $threshold;
    while(my($key, $value) = each %HASH) {
        if(@top < $keep) {
            printf "Adding %s => %d\n", $key => $value;
            push @top, [$key, $value];
            $threshold = $value if !defined $threshold or $value > $threshold;
        } elsif($value > $threshold) {
            @top = sort { $b->[1] <=> $a->[1] } @top, [$key, $value];
            printf "Inserting %s => %d\n", $key => $value;
            $threshold = $top[$keep-1][1];
            printf "New threshold: %d\n", $threshold;
            while($top[-1][1] < $threshold) {
                printf "Popping %s => %d\n", @{$top[-1]};
                pop @top;
            }
            printf "Keeping %d items\n", scalar @top;
        } elsif($value == $threshold) {
            printf "Adding %s => %d\n", $key => $value;
            push @top, [$key, $value];
        }
    }
}

use Data::Dumper;
$Data::Dumper::Terse = 1;
print Dumper \@top;