A little more generally you could:

  my @arr = ('A'..'Z');
  my $n = 2 ;
  my $combo = join '', map { $arr[rand @arr] } (1..$n) ;
  print $combo . "\n";
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which will give you any length string you like.

Whatever you do, don't do:

  my @arr = ('A'..'Z');
  my $n = 2 ;
  my $combo = $arr[rand @arr] x $n ;
  print $combo . "\n";
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however plausible it might appear :-(

Of course it doesn't work, you say, it's obvious... However, I have, ahem, a friend who once (once was enough):

  my @arr = ([]) x $count ;
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and had a lot of fun (tm) working out why their program behaved as oddly as it did !