Web Standards Programmer's Reference: HTML, CSS, JavaScript®, Perl, Python®, and PHP

25.6. Control Structures

Like other languages, Perl supports many different control structures that can be used to execute particular blocks of code based on decisions or repeat blocks of code while a particular condition is true. The following sections cover the various control structures available in Perl.

25.6.1. While and Until

The while loop executes one or more lines of code while a specified expression remains true. The while loop has the following syntax:

while (<expression>) {
  # statement(s) to execute
}

Because the <expression> is evaluated at the beginning of the loop, the statement(s) will not be executed if the <expression> is false at the beginning of the loop. For example, the following loop will execute 20 times, each iteration of the loop incrementing x until it reaches 20:

my $x = 0;
while ($x <= 20) {  # do until $x = 20 (will not execute when x = 21)
  $x++;  # increment x
}

The until loop is similar to the while loop except that the loop is executed until the expression is false:

until (<expression>) {
  # statement(s) to execute while expression is true
}

25.6.2. For

The for loop executes statement(s) a specific number of times and is governed by two expressions and a condition:

for (<initial_value>; <condition>; <loop_expression>) {
  # statement(s) to execute
}

The <initial_value> expression is evaluated at the beginning of the loop; this event occurs only before the first iteration of the loop. The <condition> is evaluated at the beginning of each loop iteration. ...