[
        # First loop: all item indices
        [ 0..$#items ],

        # @items-1 subsequent loops:
        ( sub {
            # If we need more items:
            $sum[@_] < $target
                # then get more (unique) item indices
                ? [ ($_+1)..$#items ]
                # else don't get more items
                : []
        } ) x $#items,
    ],

##</code><code>##

for $_ (  0..$#items  ) {
    # ...
    for $_ (  @{ $sum[@_] < $target
        ? [ ($_+1)..$#items ] : [] }
    ) {
        # ...
        for $_ (  @{ $sum[@_] < $target
            ? [ ($_+1)..$#items ] : [] }
        ) {
            # ...
        }
    }
}

##</code><code>##

        OnlyWhen => sub {
            # Compute sum of selected items as
            # sum of @_-1 items plus last item:
            $sum[@_]= $sum[$#_] + $items[$_[-1]];

            # Return subsets that match desired sum:
            return $sum[@_] == $target;
        },

##</code><code>##

@_= ();
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 @_;
}

##</code><code>##

$sum[@_] < $target ? [ ($_+1)..$#items ] : []

##</code><code>##

    next   if  $target <= $sum[@_];

##</code><code>##

@_= ();
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 @_;
}

##</code><code>##

while(  my @list= @items[ $iter->() ]  ) {
    warn "$target = sum( @list )\n";
}