You are missing the keyword keys before %top_words as it has already been pointed out. Also as suggested by using an explicit reverse rather than swapping $a and $b you make your intentions clearer.

Rather than using the loop, you can just use an array slice as shown in the sample code below:

use warnings;
use strict;
use Smart::Comments;
my %words = (A  => 440, B  => 765, C  => 369, D  => 788, 
             E  => 647, F  => 216, G  => 788, 
             H  => 871, I  => 733, J  => 947, 
             K  => 471, L  => 875, M  => 982, 
             N  => 314, O  => 564, P  => 178, 
             Q  => 212, R  => 487, S  => 768, 
             T  => 939, U  => 410, V  => 753, 
             W  => 765, X  => 413, Y  => 201, 
             Z  => 233, AA  => 553, AB  => 601,
             AC  => 267, AD  => 988, AE  => 140, 
             AF  => 784, AG  => 667, AH  => 987, 
             AI  => 460, AJ  => 186, AK  => 592, 
             AL  => 835, AM  => 427, AN  => 28, 
             AO  => 214, AP  => 912, AQ  => 167, 
             AR  => 421, AS  => 400, AT  => 583, 
             AU  => 250, AV  => 248, AW  => 377, 
             AX  => 850, AY  => 862, AZ  => 430, 
             AAA  => 269, AAB  => 656, AAC  => 30, 
             AAD  => 124, AAE  => 444, AAF  => 598, 
             AAG  => 915, AAH  => 276, AAI  => 9, 
             AAJ  => 831, AAK  => 522, AAL  => 753, 
             AAM  => 799, AAN  => 777, AAO  => 330);

my @top_words = (reverse sort 
                { $words{$a} <=> $words{$b} } 
        keys %words) [0 .. 19 ]; # Not [1 .. 20]!!!
### @top_words

[download]

In reply to Re: Printing keys from a hash by value? by ricDeez
in thread Printing keys from a hash by value? by jms53

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.