Programming Perl, 4th Edition

Boolean Context

Another special don’t-care scalar context is called Boolean context. Boolean context is simply any place where an expression is being evaluated to see whether it’s true or false. When we say “true” and “false” in this book, we mean the technical definition that Perl uses: a scalar value is true if it is not the null string "" or the number 0 (or its string equivalent, "0"). A reference is always true because it represents an address that is never 0. An undefined value (often called undef) is always false because it looks like either "" or 0, depending on whether you treat it as a string or a number. (List values have no Boolean value because list values are never produced in scalar context!)

Because Boolean context is a don’t-care context, it never causes any scalar conversions to happen, though of course the scalar context itself is imposed on any operand that cares. And for many operands that care, the scalar they produce in scalar context represents a reasonable Boolean value. That is, many operators that would produce a list in list context can be used for a true/false test in Boolean context. For instance, in list context such as that provided by the unlink operator, an array name produces the list of its values:

unlink @files;      # Delete all files, ignoring errors.

But if you use the array in a conditional (that is, in a Boolean context), the array knows it’s in scalar context and returns the number of elements in the array, which conveniently is true so long as ...