There could be more than one longest words having the same length, you probably want all of them:

use strict;

my @words = ("one", "two", "123456789012345", "three", "four", "sumofa
+llnumbers", "five", "six", "seven", "eight");
my @sorted_words = sort {length($b) <=> length($a)} @words;
[download]

my @longest_words = grep (/.{$#sorted_words}/, @words);

print join(",", @longest_words);
[download]