Programming Perl, 4th Edition

Terms and List Operators (Leftward)

Any term is of highest precedence in Perl. Terms include variables, quote and quote-like operators, most expressions in parentheses, brackets or braces, and any function whose arguments are parenthesized. Actually, there aren’t really any functions in this sense, just list operators and unary operators behaving as functions because you put parentheses around their arguments. Nevertheless, the name of Chapter 27 is Functions.

Now listen carefully. Here are a couple of rules that are very important and simplify things greatly, but may occasionally produce counterintuitive results for the unwary. If any list operator (such as print) or any named unary operator (such as chdir) is followed by a left parenthesis as the next token (ignoring whitespace), the operator and its parenthesized arguments are given highest precedence, as if it were a normal function call. The rule is this: if it looks like a function call, it is a function call. You can make it look like a nonfunction by prefixing the parentheses with a unary plus, which does absolutely nothing, semantically speaking—it doesn’t even coerce the argument to be numeric.

For example, since || has lower precedence than chdir, we get:

chdir $foo    || die;       # (chdir $foo) || die
chdir($foo)   || die;       # (chdir $foo) || die
chdir ($foo)  || die;       # (chdir $foo) || die
chdir +($foo) || die;       # (chdir $foo) || die

but because * has higher precedence than chdir, we get:

chdir $foo * 20; # chdir ($foo * 20) chdir($foo) ...