#!/usr/bin/perl
use strict;
use warnings;

my @array = 0 .. 30;
my $next = by_groups_of( 3, \@array );

while ( my $group = $next->() ) {
    $group->[1] = 'foo';
}
print "$_\n" for @array;

sub by_groups_of {
    my ($by, $list) = @_;
    return sub { () } if ! $by || $by =~ /\D/ || ! @$list;

    my $pos = 0;
    my $done;

    return sub {
        return () if $done || $pos > $#$list;
        my $start = $pos;
        my $stop = $pos + $by - 1 > $#$list ? $#$list : $pos + $by - 1;
        $pos += $by;
        $done = 1 if $stop == $#$list;
        return sub { \@_ }->( @$list[ $start .. $stop ] );
    }
}