Perl numeric data types. 
Type              Example
Integer           123 
Floating          1.23 
Scientific        1.23E4 
Hex               0x123 
Octal             0123 
Binary            0b101010 
Underlines        1_234_567 

The underlined formats digits in a number in groups, such as 1,234,567 
$variable1 = 1_234_567;
          
