In the following layout, each letter is to be replaced by a decimal digit. As there are more than ten letters, there's obviously repetition of digits.

  A B  
C D E F
G H I J
  K L  

The digits can be considered as forming eight numbers, read left to right and top to bottom. Four have four digits each and four have two digits each.

Seven of these eight numbers are perfect squares. Only AB is not.

What are the digits to replace the letters?