#!/usr/bin/perl
use warnings; 
 $string="ATATGCGCAT";
###########################################
# Output to a TEXT File: 
###########################################
$output="Results .txt";
open (my $fh,">",$output) or die"Cannot open file
  '$output'.\n";  
##################################### 
# To break into 4-letter fragments: 
##################################### 

while ($string=~ /(.{4}?)/ig)  
 {$four=$&;     
  @sw=$four=~ /[ATGC]{1}/igs; 
 foreach my $single (@sw) { 
####################################################
# To extract single letter & append perd to single: 
####################################################
 $perd="%d";
 $mod_four=$single.$perd; # concatenation
 push  @new_four,$mod_four;  
  $new_four = join ('',@new_four);
# To produce all possible combinations without changing positions: 
for  $a (1 .. 2) { # a  has 2 levels:
 for  $t (1 .. 2) { # t has 2 levels:
  for  $g (1 .. 2) { # g has 2 levels:
   for  $c (1 .. 2) { # c has 2 levels: 

 $combi=sprintf($new_four,$a,$t,$g,$c,3-$a,3-$t,3-$g,3-$c); 
print"~$combi\n"; 
print $fh "~$combi\n";        
   }
  }
 }
}
   } # 2nd foreach closes: 
   
} # 1st while closes: 
print"~"; 
print"\n";  
print $fh "~"; 
print $fh "\n";  
close $output;     
exit;