Programming Perl, 4th Edition

while and until Statements

The while statement repeatedly executes the block as long as EXPR is true. If the word while is replaced by the word until, the sense of the test is reversed; that is, it executes the block only as long as EXPR remains false. The conditional is still tested before the first iteration, though.

The while or until statement can have an optional extra block: continue. This block is executed every time the block is continued, either by falling off the end of the first block or by an explicit next (a loop-control operator that goes to the next iteration). The continue block is not heavily used in practice, but it’s in here so we can define the three-part loop rigorously in the next section.

Unlike the foreach loop we’ll see in a moment, a while loop has no official “loop variable”.^[75] You may, however, declare variables explicitly. A variable declared in the test condition of a while or until statement is visible only in the block or blocks governed by that test. It is not part of the surrounding scope. For example:

while (my $line = <STDIN>) {
    $line = lc $line;
}
continue {
    print $line;   # still visible
}
# $line now out of scope here

Here, the scope of $line extends from its declaration in the control expression throughout the rest of the loop construct, including the continue block, but not beyond. If you want the scope to extend further, declare the variable before the loop.

^[75] A consequence of this is that a while never implicitly localizes any variables in ...