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