my %line = ('A' =>1,
            'B' =>1,
            'C' =>1);

my @tojoin = qw (W X Y Z);


foreach  my $line ( keys %line  )
{
   print "$line\n";

   foreach my $tojoin ( @tojoin  )
   {
      my $nstr = $line.$tojoin;
      $line{$nstr} = 1;
   }           
     last if ($line eq 'AYW');

}               

##</code><code>##

A => 1,
B => 1,
C => 1,
AW => 1,

.... until

AYW => 1