use strict;
use warnings;

my @array = 0 .. 30;
my $rc_next = by_groups_of( 3, [ \(@array) ] );

while ( my $ra_group = $rc_next->() ) {
    last unless @$ra_group;
    ${$ra_group->[1]} = 'foo';
}

print "$_\n" for @array;

sub by_groups_of {
    my ($by, $ra_list) = @_;

    $by ||= 0;
    $ra_list ||= [];
    my $rc_return = sub { [] };

    if ($by && $by !~ /\D/ && @$ra_list)
    {
        my $pos = 0;
        my $done;

        $rc_return = sub {
            return [] if $done;
            my $start = $pos;
            my $stop = $pos + $by - 1 > $#{$ra_list} ? $#{$ra_list} : $pos + $by - 1;
            #$pos += $by - 1;   # ?? bug ?? - changed to "$pos += $by;"
            $pos += $by;
            $done = 1 if $stop == $#{$ra_list};
            return [ @{$ra_list}[$start .. $stop] ];
        };

    }

    return $rc_return;
}