Update: make code more readable

#!/usr/bin/perl -wl
use strict;

my @data =
  (
   [qw(a b c)],
   [qw(d e)],
   [qw(f g h)],
   [qw(i)],
   [qw(j k)],
   [qw(l)],
   [qw(m)],
  );

my @pos = (0) x @data;

sub iterator {
    my $i = @pos;
    while ($i--) {
        $pos[$i] = ($pos[$i] + 1) % @{$data[$i]}
          and last;
    }
    return  scalar grep $_, @pos;
}

sub items {
    my ($n, $str) = @_;
    return if length $str < $n;
    return substr($str, 0, 1) if $n == 1;
    return $str if $n == length $str;
    my @items = items($n, substr($str, 1));
    for my $i (1 .. length($str) - 1) {
        push @items,
             map { substr($str, 0, 1) . $_ }
               items($n - 1, substr($str, $i));
    }
    return @items;
}

do {
    for (4 .. @data) {
        print
          for items($_,
                    join '',
                    map { $data[$_]->[$pos[$_]] }
                    0 .. $#data
                   );
    }
}
while iterator();
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