The easiest way would be like this:

use strict;
my %hash = (
    a_one => 1, 
    b_two => 2,
    c_three => 3,
    d_four => 4,
    e_five => 5,
    f_six => 6,
    g_seven => 7,
    h_eight => 8,
    i_nine => 9,
    j_ten => 10
); 
my $pairs = (keys %hash);
my $counter = 0;
my $amount_to_print = int(0.20 * $pairs);

foreach my $key (reverse sort keys %hash) {
  if ($counter < $amount_to_print) {
    print $key, ' ', $hash{$key}, "\n";
  }
  else {
    last;
  }
  $counter++;
}
[download]

This will print:

j_ten 10
i_nine 9
[download]

Maybe there's some better solution?