After the loop block, you can use the continue clause if you need to specify some additional statements to be executed after the current iteration of the block.

The Perl while statement can be used with a do block, too. An alternative to the while statement is the until statement, which executes the block only as long as the condition is evaluated false.

LABEL while (EXPR) BLOCK continue BLOCK

EXPR is a boolean conditional expression that it will be evaluated before the execution of the block. The Perl while statement will repeatedly execute the block as long as EXPR is true.

The continue block is executed before the successive reevaluations of the test condition, excepting the case when the main block is exited by the last loop control. It provides a block of statements to be executed after the current iteration is terminated.

$day = "Monday";
while ($day eq "Sunday") {
  print "It's Sunday\n";
} 
print "It's Monday\n";
# displays It’s Monday

while(1) {
  $k++;
}
# the next statement will never be reached:
$k = 0;

If you want to exit from an infinite loop, you can use the last loop control, as shown in the next example:

$k = 0;
while(1) {
  last if $k == 1000;  
  $k++;
}
print "k = $k\n";
# it displays k = 1000;

# initialize a hash (associative array)
%numbers = (1 => 'one ', 2 => 'two ',
            3 => ' three', 4 => ' four');
while(my $key = each %numbers) {
  $numbers{$key} = $key * 10;;
}
continue {
  print "Key: $key, Value: $numbers{$key}\n";
}

Key: 4, Value: 40
Key: 1, Value: 10
Key: 3, Value: 30
Key: 2, Value: 20

We modified the value of each key in the main block of the Perl while statement and after that we print the (key value) pairs of the hash structure in the continue block. Note that the elements of the hash structure are not printed in the natural order I initialized the hash because Perl has its internal order in which it stores the elements. If you want to sort the elements by key or by values, see Sort or order a hash.

# initialize a square numbers array
@sqNoArray = ();
FOR: {
  my $k = 1; # the initialization
  while($k <= 10)  # the condition
  {
    # append an element to the array
    push(@sqNoArray, $k**2);
  }
  continue { $k++; } # the re-initialization
} # end of FOR block

# print the @sqNoArray
print "@sqNoArray\n";
# output: 1 4 9 16 25 36 49 64 81 100

#initialize an array
@colors = ();
while(<STDIN>) {
  chomp;
  push(@colors, $_);    
}
print "Colors: @colors\n";

You can rewrite the above snippet code without using the special variable $_, in the following way:

#initialize an array
@colors = ();
while($line = <>) {
  chomp $line;
  push(@colors, $line);    
}
print "Colors: @colors\n";

A possible output after running any of the codes above might be:

red
yellow
green
Ctrl/Z 
Colors: red yellow green

At this point you may wonder how to use Perl while statement with <> when the @ARGV array is not empty. When you are running a Perl script, the elements of the @ARGV array will be added from the command line. The next example is meant to be run for a Windows platform and from the command line. On my computer Perl is installed on the C:\ drive. Please take a look at the following table columns:

I created three files: Colors.pl that represents the name of my script and Colors1, Colors2 with the lines indicated in the above table. After I created the three files in the \bin directory of Perl, I ran the Colors.pl script from the command line and I’ve got the following output:

C:\Perl\bin\perl Colors.pl Colors1 Colors2
red
green

white
blue
yellow

• the @ARGV array contains two elements, respectively the file names Colors1 and Colors2
• the diamond operator from the while statement will start reading all the lines from Colors1 file and it will continue with Colors2 file. Each line read from the files will be printed. After finishing reading the files, the Perl while loop will end.

@ARGV = qw(Colors1 Colors2);
while(<>) { print(); }

Please take a look at the following script example:

$count = 0;
while($count++ < 3) {
  chomp(my $line = <STDIN>);
  last if $line =~ /quit/;
  next if $line =~ /while|until/i;
  redo;
}
# end of the loop
print "Count = ", --$count, "\n";

this is while Perl statement
if else
until statement
unless - this is not a loop
something else
quit
Count = 2

• well, in the first line of code we initialize the $count scalar variable with 0 – we are using this variable to count the number of lines read from STDIN that match the while or until words
• in the condition expression of the Perl while statement we increment the $count variable and we’ll leave the loop after the $count value becomes greater than or equal to 3
• in the line three of code we read a line of text from STDIN, we discard the trailing newline and we store the result in the $line scalar variable
• we use the last looping control to leave the loop if we read 'quit' from STDIN
• if we case insensitively match the word while or until in the $line variable, we use the next looping control to reiterate the loop by skipping the remainder of the code block
• if we don’t match neither while nor until, we’ll reexecute the code block without the reevaluating of the test condition of the Perl while statement
(redo looping control)
• after ending the loop, we’ll print the $count scalar variable value

STATEMENT while (EXPR)

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