• while statement checks the test condition before executing the block
• do-while statement checks the test condition after executing the block.

This free short tutorial will show you some useful ways to use do-while statement in Perl scripts.

Let’s look at the syntax form of this statement, first:

do BLOCK while (EXPR);

• The condition is evaluated after executing the block so the block will be executed at least once, unlike the while statement where if the condition is false initially, the block will be not executed at all
• Don’t think of do as a loop block; here the keyword do means merely a term used in an expression rather than a loop block. And because the do block is not a looping block, you can’t use within the looping controls: next, last and redo

$count = 0;
do {
  print "Password: ";  
  chomp($psw = <STDIN>);
} while ++$count < 3 && $psw ne "1qaz"; 
 
if($psw eq "1qaz") {
  print "Password OK ";
} else {
  print "Wrong password\n";
}

Password: 1
Password: 2
Password: 3
Wrong password

do {
  ++$var;
  print "$var ";
} while $var <= 5;
print "\n";
# it displays 1 2 3 4 5 6

If you don’ feel like testing the condition after the block by using Perl do-while statement, you can initialize an iteration variable and use while statement instead, in order to execute your block at least once. You can rewrite the previous example as follows:

$var = 0;
while ($var <= 5){
  ++$var;
  print "$var ";
} 
print "\n";

As I mentioned before, the do block doesn’t behave like a loop block, therefore you can’t use any of the control looping within the block. Unfortunately you may but the result after the execution of your code will be unpredictable. You know, sometimes Perl let you do a lot of things and if you do not do them in the appropriate way, it will take you a lot of time to debug and correct your script code.

But there is still a way to use the looping controls with the do construct. By instance, you can use the next and redo controls by putting a bare block inside the do block, like in the following snippet code:

my $a = 1; 
do 
{
  # the do block
  {
    # the bare block
    next if $a > 7;  
    print "$a ";
  }
} while $a++ < 10;
print "\na = $a\n";

1 2 3 4 5 6 7
a = 11