What's the coefficient of "aardvark" and "dark"? "aardvark" and "arbitrary"? Even http://en.wikipedia.org/wiki/Dice%27s_coefficient doesn't clarify this.

Not sure what you want to do with the coefficients, so I made stuff up:

use strict;
use warnings;
$| = 1;
print "Enter word: ";
chomp(my $word = <STDIN>);
my @pairs = $word =~ /(?=(..))/g;

my $matcher = qr/(?=(@{[join "|", @pairs]}))/;

my %coef;

open my $dict, "<", "/usr/share/dict/words"
   or die "Couldn't open dictionary: $!";
while (my $dictword = <$dict>) {
   chomp($dictword);
   # skip proper nouns and anything with a non-letter
   next if $dictword =~ /[^a-z]/;
   my $matches = () = $dictword =~ /$matcher/g;
   my $coef = 2 * $matches / (@pairs + length($dictword)-1);
   push @{$coef{$coef}}, $dictword;
}

print "Top coefficients for $word:\n";
for my $coef ((sort { $b <=> $a } keys %coef)[0..4]) {
   next if ! $coef;
   print "$coef: ", join " ", @{$coef{$coef}}, "\n";
}
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