In Perl, a scaler is synonymous with a variable. Regardless if you use the label scaler or variable, you are probably referencing the same thing. A scaler/variable is identified by the $ character, and the scaler/variable contains data, such as an integer (numbers), text, special characters, or even an entire file. In this example, $foo is the scaler the text "Hello World" would be the data in the scaler/variable.
One of the nice things about Perl is that unlike some other languages, such as Java, you do not need to specify the scaler/variable type. For example, in Java, when creating a variable in Java, you would need to specify the variable as an int (integer). You do not need to worry about this in Perl, which is nice.
my $foo = "Hello World";