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

my $length = 2;

my @motif = ( '' );

for my $i ( 1 .. $length ) {
    @motif = map {
        my $motif = $_;
        map $motif . $_, qw( a t c g );
    } @motif;
}

print Dumper \@motif;