use strict;
use warnings;            
use Data::Dumper;        

my $l = 6;    #Motif Length
my $d = 2;               
my $str = 'TTTCGG';      
my @nucs = qw/A T C G/;  

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


foreach  my $oligo ( @enum  ) {
    
    if ( hd ($oligo,$str) <= $d ) {
       print "$oligo\n"; 
    }
}               



sub hd {
    return ( $_[0] ^ $_[1] ) =~ tr/\001-\255//;
}


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