my @arr = ('A' .. 'Z');
my @combo;

my $n = 2;

# keep at it till we get $n random values in one pass
do {
    @combo = ();

    my $i = 0;
    rand(1) < $n / ++$i && splice(@combo, rand(@combo), $n <= @combo, $_) for @arr;
} while @combo < $n;

print "@combo\n";