It’s important to highlight that both looping statements – while and until – will evaluate the test condition before to execute the statement body, so if the condition is initially false (for the while statement) or true (for the until statement), the block of the statement will be skipped entirely.

The syntax forms of the two statements are almost identically, you can replace while with until and you have the syntax forms of the until statement – of course they differ in function of the test condition result (true or false). So I heartily recommend to check out my while link first and after that revisit this page.

You can also use the last, next and redo looping controls in order to change the usual behavior of the Perl until loop iteration. These three controls allow you to control the flow of execution in the until block.

In the same time you can attach a continue block to the Perl until statement if you need to execute some piece of code after the current iteration.

LABEL until (EXPR) BLOCK continue BLOCK

Either the LABEL or the continue BLOCK are optional. EXPR is a conditional expression that will be evaluated before the current iteration of the loop.

You’ll find below a few ways to use the Perl until statement.

$count = -100;
until(0) {
  last if $count >= 100;
  $count++;
}
print "count = $count\n";
# it displays count = 100;

until ($line = <STDIN>) {
    print "I am inside the main block\n";
} 
continue {
  print "I am inside the continue block\n";
}

chomp $line;
print "I typed: $line\n";

First line
I typed: First line

Well, is not entirely true what I said before, the next code snippet shows you another way to use until in connection with the <> diamond operator:

until($exit =~ /yes/i ) {
  print "Exit the loop? (yes/no): ";
  chomp($exit = <STDIN>);
}  

Do you exit the loop? (yes/no): No, I don't
Do you exit the loop? (yes/no): Yes, I do

$count = 10;
until($count <= 0) {
  last if $count < 3;
  next if $count%2 == 0;
  $count--, redo if $count == 7;
  print "$count ";
}
continue {
  $count--;
}
print "\n";
# it displays 9 5 3

Note the using of the remainder operator and the next looping control that allowed me to skip the even numbers.

The next control, before to reiterate the loop and test the conditional expression, will execute the statements from within the continue block.

The last control will jump out of the main block without executing the continue block.

# initialize an array
$count = 0;
@numbers = qw(one two three four five);
until(!@numbers) {
  $count++;
  shift(@numbers); 
}
print "$count elements were deleted!\n";
# it displays: 5 elements were deleted!

• unshift – insert an element at the beginning of an array
• push - adds an element at the end of an array
• pop – removes the last element of an array

$count = 100;
# initialize a hash
%numbers = (1 => "one", 2 => "two",
            3 => "three", 4 => "four",
            5 => five);
until(!%numbers) {
  $count--;
  %numbers = () if $count == 0;    
}	
print "count = $count\n";
# it displays: count = 0

STATEMENT until (EXPR)

$count = 10;
print $count, " " until --$count <=0;
print "\n";
# it displays: 9 8 7 6 5 4 3 2 1
print "count = $count\n"  # displays count = 0

If the conditional expression is false, the $count variable will be printed and the loop iteration will continue; otherwise the loop will be finished and Perl will execute the third line of this code snippet.