This should get you started - extract the words into a stack (array). Then in a loop: pull the first word off the stack, process it, check that there is a word remaining, combine with the previous top word and process, check that there is a third word, combine and process.

use warnings;
use strict;

use Text::ExtractWords;

my $text = do {local $/; <DATA>}; # Slurp the text
my @words;
my %hash;

words_list (\@words, $text);

while (@words) {
    ++$hash{$_ = shift @words};
    ++$hash{"$_ $words[0]"} if exists $words[0];
    ++$hash{"$_ $words[0] $words[1]"} if exists $words[1];
}

print "$_: $hash{$_}\n" for sort keys %hash;

__DATA__
Three blind mice.  Three blind mice.  See how they run.  See how they 
+run.
The butcher's wife came after them with a knife, three blind mice.
[download]

Prints:

a: 1
a knife: 1
a knife three: 1
after: 1
after them: 1
after them with: 1
blind: 3
blind mice: 3
blind mice see: 1
blind mice three: 1
butcher's: 1
butcher's wife: 1
butcher's wife came: 1
came: 1
came after: 1
came after them: 1
how: 2
how they: 2
how they run: 2
knife: 1
knife three: 1
knife three blind: 1
mice: 3
mice see: 1
mice see how: 1
mice three: 1
mice three blind: 1
run: 2
run see: 1
run see how: 1
run the: 1
run the butcher's: 1
see: 2
see how: 2
see how they: 2
the: 1
the butcher's: 1
the butcher's wife: 1
them: 1
them with: 1
them with a: 1
they: 2
they run: 2
they run see: 1
they run the: 1
three: 3
three blind: 3
three blind mice: 3
wife: 1
wife came: 1
wife came after: 1
with: 1
with a: 1
with a knife: 1
[download]