my @slot = qw[ A A B B C C B B ];
my %slot_mask;

# Create a hash based on the input names. Each hash entry
# contains a single binary sequence for the "slot" numbers
# that are specified earliner in @slot
foreach (0..$#slot)
{
     # Create a "blank" scalar if none is defined already
     unless (defined($slot_mask{$slot[$_]})
     {
          $slot_mask{$slot[$_]} = '';
     }

     # Set this particular bit to 1 using an assignment
     vec($slot_mask{$slot[$_]}, $_, 1) = 1;
}

# Retrieval, as per your example, for the 1st slot
if ($slot_mask{A} & 0b00000010)
{
      print "1st is A...\n";
}

# Same idea but with the shift operator that is less prone
# to typographical errors
if ($slot_mask{A} & (1 << 1))
{
      print "1st is A...\n";
}

# And likewise for the 4th or what have you, but this time
# using the vec() function
if (vec($slot_mask{A}, 4, 1))
{
      print "4th is A...\n";
}