#!/usr/bin/perl -l
use warnings;
use strict;

my $data = 'Douglas built five Douglas World Cruisers to attempt his first flight to Buenos Aires. These were the predecesors of the modern AH-64D and AH-64D Apache.';

# What do you call a capitalized "word"?
my $cap_word = qr/[A-Z][\w-]*/;

my @clusters = $data =~ /
  #(?<!\.\s)           # Ignore words at begining of sentences?
  (
    $cap_word          # Capitalized word, followed by any number
    (?:\s+$cap_word)*  # of other cap words (separated by spaces)
  )
/gx;

# Update: Oh, you wanted the largest...
print "Largest cluster: ", (sort { length $b <=> length $a } @clusters)[0];