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

my @lines = <DATA>;

my (%words, %count);
@words{map {split ' '} @lines} =1;
my @uniq = keys %words;                 # create list of unique words

foreach my $i (0 .. $#uniq - 1) {
        foreach my $j ($i+1 .. $#uniq) {

                # count the pairs
                do { $count{"$uniq[$i] $uniq[$j]"}++ if /$uniq[$i]/ and /$uniq[$j]/ } foreach @lines;   

        }
}


print   map "$_ : $count{$_}\n",                # print the pairs in sorted order
        sort {$count{$a} <=> $count{$b}}
        keys %count;






__DATA__
dog monkey cat
cat ball stone
monkey iron cat zoo