The following is relatively simple and efficient. The result is the top K pairs, with preference given to higher numbers in the event of a tie. Integer data from 1 to N allow us to use an array in place of a hash (the indices of the array form the required "set"):

# preconditions:
my $elem = 1;
my $k = 3;
my $n_arrays = [map [split], <DATA>];

# algorithm:
my @pairs = (0) x (@$n_arrays + 1);
for (@$n_arrays) {
    next unless grep $_==$elem, @$_[0..$elem];
    $_ != $elem and $pairs[$_]++ for @$_;
}
my @top_k = (sort{$pairs[$a]<=>$pairs[$b]} 1..$#pairs)[reverse -$k..-1
+];
print "@top_k\n";
__END__
2 4 5 7 8 10 
1 2 5 6 7 9 
2 6 7 8 9 10 
1 3 5 10 
1 3 4 5 6 8 9 
1 2 4 6 
1 2 4 5 7 10 
1 3 4 6 7 8 9
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