Detect the Nearly Perfect Licence Plates

Jelly, 29 28 30 bytes

+1 byte to fix a bug spotted by ChristianSievers (incorrectly dealing with substrings of only zeros) +1 byte to fix false positives for "0", "00", ... found during above fixing (0 is a perfect square).

i@€ØAS;Ʋ$
e€ØAœpV€€LÐfS€P;0⁼Ç

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How?

i@€ØAS;Ʋ$ - Link 1: [letter-sum, letter-sum is perfect square?]: plate
i@€        - index of €ach char in plate [reversed @rguments] (1-based, 0 otherwise) in:
   ØA      -     uppercase alphabet
     S     - sum
         $ - last two links as a monad:
      ;    -     concatenate with:
       Ʋ  -         is square?

e€ØAœpV€€LÐfS€P;0⁼Ç - Main link: plate                        e.g. "11BB2"
    œp              - partition plate at truthy values of:
e€                  -     is in? for €ach char in plate:
  ØA                -         uppercase alphabet                   [['1','1'],[''],['2']]
      V€€           - evaluate for €ach for €ach                   [[1,1],[],[2]]
          Ðf        - filter keep:
         L          -     length                                   [[1,1],[2]]
            S€      - sum each                                     [2,2]
              P     - product                                      4
               ;0   - concatenate a zero                           [4,0]
                  Ç - last link (1) as a monad (taking plate)      [4,1]
                 ⁼  - equal? (non-vectorising)                     0

MATL, 36 34 33 35 bytes

3Y432YXU"@V!Usvp]GlY2&msy=wtQ:qUm~v

Try it at MATL Online

Explanation

        % Implicitly grab input as a string
3Y4     % Push the predefined literal '[A-Za-z]+' to the stack
32      % Push the literal 32 to the stack (ASCII for ' ')
YX      % Replace the matched regex with spaces (puts a space in place of all letters)
U       % Convert the string to a number. The spaces make it such that each group of
        % of consecutive digits is made into a number
"       % For each of these numbers
  @V!U  % Break it into digits
  s     % Sum the digits
  v     % Vertically concatenate the entire stack
  p     % Compute the product of this vector
]       % End of for loop
G       % Explicitly grab the input again
lY2     % Push the predefined literal 'ABCD....XYZ' to the stack
&m      % Check membership of each character in the input in this array and 
        % return an array that is 0 where it wasn't a letter and the index in 'ABC..XYZ'
        % when it was a letter
s       % Sum the resulting vector
y       % Duplicate the product of the sums of digits result
=       % Compare to the sum of letter indices result
w       % Flip the top two stack elements
Q       % Add one to this value (N)
t:      % Duplicate and compute the array [1...N]
q       % Subtract 1 from this array to yield [0...N-1]
U       % Square all elements to create all perfect squares between 1 and N^2
m~      % Check to ensure that N is not in the array of perfect squares
v       % Vertically concatenate the stack.
        % Implicitly display the truthy/falsey result

Python 2, 120 118 bytes

s=t=p=0;r=1
for n in input():
 h=int(n,36)
 if h>9:s+=h-9;r*=t**p
 p=h<10;t=(t+h)*p
print(s==r*t**p)&(int(s**.5)**2<s)

Try it online!

Interprets each character as a number in base-36 (h). Converts to decimal and adds to the sum if h>9 (meaning it's a letter), otherwise adds to a variable which gets multiplied to form the running product later.