% perl -E '@a = 1 .. 4; for my $x (@a) { %set = map { $_ => 1 } @a; delete $set{$x}; say "$x, $_" for keys %set; }'
1, 4
1, 3
1, 2
2, 4
2, 1
2, 3
3, 4
3, 1
3, 2
4, 1
4, 3
4, 2
%