#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $l = 2;    #Motif Length
my @nucs = qw/A T C G/;

my @enum = enum( $l, \@nucs );
print Dumper \@enum;

sub enum {
    return @{$_[1]} unless --$_[0];
    map { my $nuc = $_; map { $nuc . $_ } @{$_[1]} } enum( $_[0], $_[1]);
}