Here's a different strategy, based on using split() to separate a string into pieces using non-capitalized words as separators, then discarding empty parts. This scheme needs some tweaking to honor sentence boundaries, but the general trick is a useful one in situations like this.

#!/usr/bin/perl -w
use strict;

my $source = join('', <DATA>);
my @capgroups = grep { $_ } split(/(?:^|\s+)(?:[^A-Z]\S*\s*)+/, $sourc
+e);
foreach ( @capgroups ) {
    print "$_\n";
}

__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.
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