@_= ();
for $_ (  0..$#items  ) {
    push @_, $_;
    push @return, [ @_ ]   if  when( @_ );
    next   if  $target <= $sum[1];
    for $_ (  ($_+1)..$#items  ) {
        push @_, $_;
        push @return, [ @_ ]   if  when( @_ );
        next   if  $target <= $sum[2];
        for $_ (  ($_+1)..$#items  ) {
            push @_, $_;
            push @return, [ @_ ]   if  when( @_ );
            # ...
        } continue {
            pop @_;
        }
    } continue {
        pop @_;
    }
} continue {
    pop @_;
}