A scalar is the simplest kind of data that Perl manipulates. A scalar is either a number (like 4 or 3.25e20) or a string of characters (like "hello" or the Gettysburg Address). Although you may think of numbers and strings as very different things, Perl uses them nearly interchangeably, so we'll describe them together.
A scalar value can be acted upon with operators (like plus or concatenate), generally yielding a scalar result. A scalar value can be stored into a scalar variable. Scalars can be read from files and devices and written out as well.