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

print for generate(4, 
                   [qw(a b c)], [qw(d e)], [qw(f g h)], ["i"], 
                   [qw(j k)], ["l"], ["m"]);

sub generate {
    my $min = shift;
    my @non_empty = grep @$_, @_;
    return if $min > @non_empty;
    work($min, @non_empty);
}

sub work {
    my $min = shift;
    return "" if !@_;
    my $first = shift;
    return map { 
        my $n = $_; 
        length() < $min ? () : $_, map $_.$n, @$first;
    } work($min-1, @_);
}