my %count_of;

while (<DATA>) {
    chomp;
    # sort the words so I don't have "cat-dog" and "dog-cat"
    my @words = sort split;
    foreach my $i ( 0 .. $#words - 1 ) {
        foreach my $j ( $i+1 .. $#words ) {
            $count_of{ $words[$i] . q{-} . $words[$j] }++;
        }
    }
}

my $top_pair = ( sort { $count_of{$a} <=> $count_of{$b} } keys %count_of )[-1];
print "$top_pair ($count_of{$top_pair})\n";

__DATA__
dog monkey cat
cat ball stone
monkey iron cat zoo