#! perl -slw
use strict;

sub pairs {
    map {
        my $i = $_;
        map [ @_[ $i, $_ ] ], $i+1 .. $#_;
    } 0 .. $#_;
}

my %pairCounts;

++$pairCounts{ "@$_" } for map pairs( sort split ), <DATA>;

print "@$_" while @{ $_ = [ each %pairCounts ] };


__DATA__
dog monkey cat
cat ball stone
monkey iron cat zoo