Scope of for loop diminishes after the block in Perl

use strict;
use warnings;
use feature qw( say );

my @aa = ("A", "C", "D", "E", "F", "G", "H", "I", "K", "L", "M", "N", "P", "Q", "R", "S", "T", "V", "W", "Y");

my $pair;
for my $n1 (@aa) {
   for my $n2 (@aa) {
     $pair = $n1 . $n2;    
        	   
   }
}
my $k1=split("' '", $pair);
print $k1;

# these are the 20 amino acids and i have to make 400 possible combination of two

# amino acid(di amino acid) like this: AA AC AD ...... CA CC CD CE CF CG.....YA.....YY

# I TRIED TO MODIFY THE CODE but the scope of "for" loop diminishes # when it ends.why it is ending there only since i made it global #only two Y's as output