I'm assuming you want to count how many of each word pair is in the document, and rank the words by number found. I'm also assuming that a line break is always the end of a sentence, and that you want periods to signify the end of a sentence.

use strict;

my (@lines, @words, $i, $pair, %hash);
foreach (<DATA>) {
    $_ =~ s/[^\w\. ]//g;          # Remove unneeded characters
    $_ =~ s/  +/ /g;              # Many spaces to one
    $_ =~ s/ ?\. ?(\. ?)*/\./g;   # Boundaries with . change to .
    $_ =~ s/^ //; $_ =~ s/ $//;   # Spaces at start and end removed
    $_ = lc($_);                  # Lowercase
    @lines = split(/\./, $_);     # Split on sentence boundaries
    foreach (@lines) {            # Get words for each sentence
        @words = split(/[\. ]/, $_);
        for ($i = 0; $i < $#words; $i++) {   # For each word pair
            $pair = @words[$i] . ' ' . @words[$i+1];
            $hash{$pair}++;       # Increment count for word pair
        }
    }
}
foreach (sort {$hash{$b} <=> $hash{$a}} keys %hash) {
    print $_ . ' ' . $hash{$_} . "\n";
}

__DATA__
Four score and seven years ago our fathers brought forth, upon this co
+ntinent, a new nation, conceived in liberty, and dedicated to the pro
+position that "all men are created equal"

Now we are engaged in a great civil war, testing whether that nation, 
+or any nation so conceived, and so dedicated, can long endure. We are
+ met on a great battle field of that war. We have come to dedicate a 
+portion of it, as a final resting place for those who died here, that
+ the nation might live. This we may, in all propriety do. But, in a l
+arger sense, we can not dedicate -- we can not consecrate -- we can n
+ot hallow, this ground -- The brave men, living and dead, who struggl
+ed here, have hallowed it, far above our poor power to add or detract
+. The world will little note, nor long remember what we say here; whi
+le it can never forget what they did here.

It is rather for us, the living, we here be dedicated to the great tas
+k remaining before us -- that, from these honored dead we take increa
+sed devotion to that cause for which they here, gave the last full me
+asure of devotion -- that we here highly resolve these dead shall not
+ have died in vain; that the nation, shall have a new birth of freedo
+m, and that government of the people by the people for the people, sh
+all not perish from the earth.
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