Prabably because $firstNum is actually the string "7\n", that is, a 7 followed by a newline. You probably got these interactively by reading from a terminal, or you read it as a line from a file, and didnt trim the newline off

Try doing a chomp($firstNum);

And guessing you aren't a native english speaker, it is a line break. A brake is what you use on your car to stop. English is such a pain with these type of things, called homophones (meaning same-sounding). You try these ones that are all pronounced the same

toe tow

bear bare

pour paw poor pore

not knot