my $stri="abctest";

my @one=('a','b','c','d','e','f','g');
                                   
my $part;

my $lone=@one;                                   
my $first_char=substr($stri,0,1);   
#print "\n$first_char";
my $second_char=substr($stri,1,1);
#print "\n$second_char";
my $third_char=substr($stri,2,1);
#print "\n$third_char"; 

if ($first_char eq 'a' || $first_char eq 'b'){ 

  for(my $ne=0;$ne<$lone;$ne++){
    
    if ($second_char eq $one[$ne])  
     {                                    
    
      if ($third_char ne 'c' || $third_char ne 'd'){
       
         $part=substr($stri,0,2); 
         print "\nThe string in part:";
         print"\n$part";
        
        }
     }
  
   } 
 
}

 
if ($first_char eq 'a' || $first_char eq 'b'){ 

  for(my $ne=0;$ne<$lone;$ne++){
    
    if ($second_char eq $one[$ne]){    
    
      if ($third_char eq 'c' || $third_char eq 'd'){
                                       
        $part=substr($stri,0,3); 
        print "\nThe string in part:";
        print"\n$part";
      }
      
     }
  
   } 
 
}


##</code><code>##

The string in part:
ab
The string in part:
abc

##</code><code>##

if ($third_char ne 'c' || $third_char ne 'd'){
$part=substr($stri,0,2);