#!/usr/bin/perl -Twl
use strict;
use List::Util qw(max);

my @lw = longest_words(join '', <DATA>);
local $" = ', ';
print length($lw[0]), ' letters: ', "@lw";

sub longest_words {
    local $_ = $_[0];
    my %W;
    $W{$1} = length($1) while /\b(\w+)\b/sg;
    my $ll = max values %W;
    return grep { $W{$_} == $ll } keys %W;
}

__DATA__
three four five six seven eight nine foobar obsolescent
superfluous inevitable pseudohash pseudonym pseudopod