my $i;
my @ar = 0 .. 10;

($get_idx_grp, $fin_idx_grp) = mk_idx_funcs( 4, \$i, \@ar );

for ( $i = 0; $i < @ar; ++$i) {
    my @idx = &$get_idx_grp;
    $ar[ $idx[0] ] = "YYY";
    &$fin_idx_grp;
}

sub mk_idx_funcs {                       # XXX bad name
    my ( $grp_size, $iter, $array ) = @_;
    my ( @group, $group_end );
    my $first_call = 1;

    return ( sub {
            if ($first_call) {
                $first_call = 0;
                $group_end  = $$iter + $grp_size;
            }
            if ( $$iter < $group_end and $$iter < @$array ) {
                push @group, $$iter;
                no warnings;
                next unless @group==$grp_size or $$iter==@$array-1;
            }
            return @group;
        },
        sub {
            $group_end += @group;
            @group = ();
            return;
        }
    );
}