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$iter_prod = NestedLoops([
   sub { iter(qw(a b c) },
   sub { iter(qw(x y z) },
]);

my $iter = do { my $done = 0;  sub { $done++ ? undef : 1 } };
iter_prod($iter, ...) if ...;
iter_prod($iter, ...) if ...;

my @iters;
push(@iters, ...) if ...;
push(@iters, ...) if ...;