Programming Perl, 4th Edition

Simple Statements

A simple statement is an expression evaluated for its side effects. Every simple statement must end in a semicolon, unless it is the final statement in a block. In that case, the semicolon is optional—Perl knows that you must be done with the statement since you’ve finished the block. But put the semicolon in anyway if it’s at the end of a multiline block, because you might eventually add another line.

Even though operators like eval {}, do {}, and sub {} all look like compound statements, they really aren’t. True, they allow multiple statements on the inside, but that doesn’t count. From the outside, those operators are just terms in an expression, and thus they need an explicit semicolon if used as the last item in a statement.

Any simple statement may optionally be followed by a single modifier, just before the terminating semicolon (or block ending). The possible modifiers are:

if EXPR
unless EXPR
while EXPR
until EXPR
for LIST
when EXPR

The if and unless modifiers work pretty much as they do in English:

$trash–>take("out") if $you_love_me;
shutup() unless $you_want_me_to_leave;

The while and until modifiers evaluate repeatedly. As you might expect, a while modifier keeps executing the expression as long as its expression remains true, and an until modifier keeps executing only as long as it remains false:

$expression++ while –e "$file$expression";
kiss("me") until $I_die;

The for modifier (also spelled foreach if you’re trying to wear out your keyboard) evaluates ...