Some ways (by no means every possible way) to do what I understand you to want done:

>perl -wMstrict -le
"my @t = ('A' .. 'M');
 ;;
 my @ra = @t;
 print qq{@ra};
 ;;
 my $start = 5;
 for my $i (0 .. $#ra) {
   if ($i < $start) {
     $ra[$i] = 'N';
     }
   }
 print qq{@ra  \n};
 ;;
 @ra = @t;
 print qq{@ra};
 ;;
 my $end = ($start <= $#ra) ? $start - 1 : $#ra;
 for my $i (0 .. $end) {
   $ra[$i] = 'N';
   }
 print qq{@ra \n};
 ;;
 @ra = @t;
 print qq{@ra};
 $_ = 'N' for @ra[ 0 .. $end ];
 print qq{@ra \n};
 ;;
 @ra = @t;
 print qq{@ra};
 @ra[ 0 .. $end ] = ('N') x ($end + 1);
 print qq{@ra \n};
"
A B C D E F G H I J K L M
N N N N N F G H I J K L M

A B C D E F G H I J K L M
N N N N N F G H I J K L M

A B C D E F G H I J K L M
N N N N N F G H I J K L M

A B C D E F G H I J K L M
N N N N N F G H I J K L M
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