use strict;
use warnings;
my $string = "ATATGCGCAT";
my @letters = split '', $string;
for( my $i=0; $i<2**@letters; ++$i ) {
  my $b = sprintf("%010b",$i);
  $b=~tr/01/12/;
  for( my $j=0; $j<@letters; ++$j ) {
    print $letters[$j],substr $b, $j, 1;
  }
  print "\n";
  last if $i==20; # remove if you want all...
}