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

my $candidate = "the the the the the the the";
my $reference = "the cat is the on the mat";

my %cand_histogram;
my %ref_histogram;

$cand_histogram{$_}++ foreach (split(/\s+/,$candidate));
$ref_histogram{$_}++  foreach (split(/\s+/,$reference));

my %seen;
printf "%-6s %-10s %-10s\n", 'Key','Candidate','Reference';
foreach my $key ( sort {
                          $seen{$b} <=> $seen{$a} #descending word cnt
                                   or
                               $a cmp $b       #alphabetic otherwise 
                       }
                  grep {!$seen{$_}++}  # each key just once, 
                                       # but count 'em also!, 
                                       # 2 means => in both hashes
                 (keys %cand_histogram, keys %ref_histogram)
                )
{
   printf "%-6s %-10s %-10s\n", $key,
                                $cand_histogram{$key}||='0',
                                $ref_histogram{$key} ||='0';
}

__END__

OUTPUT:

Key    Candidate  Reference 
the    7          3         
cat    0          1         
is     0          1         
mat    0          1         
on     0          1