Perl Statements Menu:

Simple Perl Statements
Perl Blocks
Perl Compound Statements
Perl Control Statements
    Perl Conditional Statements
    Perl Loop Statements
    Perl Loop Controls
    Perl Jump Statements
    Perl Modifiers

We can point out that a Perl program is a sequence of expressions and statements, from the beginning to the end of the program. Actually, what I said before is not entirely true, a Perl program or script could be more complicated than this, but for the moment the above definition will help you better understand the topics described below.

A Perl statement ends with the semicolon character (;) which is used to tell interpreter that the statement was complete. You can put one or more Perl statements on the same line of a Perl script or continue a statement on several lines. Please look at the following example to see some simple Perl statements:

2+4;
$var1 = 2+4; $pi = 3.14;
$var = $var1 + 
       $pi;

to see a lot of fully commented examples that help you use the Perl statements and the Perl buit-in functions in your scripts.

The syntax format for a Perl BLOCK is as follows:

{
  statement;
  ...
  statement;  
  statement
}

See an example of a block used with a foreach statement below:

@numbers = (1, 2, 3, 4);
foreach $item(@numbers) {
  $item++;
  print "$item ";
}
print "\n";   # it prints 2 3 4 5

• precedes the block
• begins with a letter or an underscore (_) and can contain either underscores or alphanumerical characters
• ends with the colon character (:)

As a rule of thumb, I suggest you to use for labels only uppercase characters, so as not to conflict with reserved words - because Perl language is case sensitevily.

You use blocks:

• with the conditional statements like if and unless
• with the loop statements like while, for, foreach, until
• by itself, and in this case we call them bare or naked blocks

LABEL BLOCK
LABEL BLOCK continue BLOCK

If a bare block is unlabelled, we call it an anonymous block. Please take a look at the following anonymous block sample:

{
  my $var1 = 2+4; my $pi = 3.14;
  my $var = $var1 + $pi;
}

@numbers = (1, 2, 3, 4, 5);
$count = 5;

MYLABEL: {
  next if $count-- == 0;
  $count1 = $count;
  # increment each element of the array  
  $_++ foreach @numbers;
  redo;
} continue
{
  print "\@numbers = (@numbers)\n"; 
}
print "that's all\n";

@numbers = (6 7 8 9 10)
that's all

If you use last instead of next to leave the block, the continue block will be no more executed. The redo control was used to restart the block. In this example we used if and foreach statements with modifiers. To iterate through the elements of the @numbers array, we used the foreach statement with the special variable $_.

to see a lot of fully commented examples that help you use the Perl statements and the Perl buit-in functions in your scripts.

if (EXPR) BLOCK else BLOCK
LABEL while (EXPR) BLOCK
LABEL foreach VAR (LIST) BLOCK

The Perl statements are very often used as compound statements. Take a look at the following compound statement example:

# a bare block labeled MAIN
MAIN: {
  # a while with a block 
  while($line = <STDIN>){
    chomp($line);
    # some blocks with if, elsif and else
    if($line == 1){
      print "You typed 1\n";
    } elsif ($line == 2) {
      print "You typed 2\n";
    } else {
      print "You typed $line, must type 1 or 2\n";
    }
  }
}

1
You typed 1
2
You typed 2
3
You typed 3, must type 1 or 2
^Z

Inside a loop statement you can use the loop controls to alter the behavior of a Perl loop. Being a structured programming language, Perl has just an entry point for any block but it allows more ways to exit the block – like in the case you want to finish earlier the loop iterations.

You can also use the goto jump statement to modify the flow of the program and jump directly at any label in the program. Anyway, be cautious if you want to use it, especially if you want to jump inside a block or a subroutine code.

Perl allows you to write a conditional or loop statement on a single code line, by using modifiers. The statement modifier will be placed at the end of the statement you want to modify.

to see a lot of fully commented examples that help you use the Perl statements and the Perl buit-in functions in your scripts.

• if – which will execute a block only when a conditional expression is evaluated as true; if the expression is true, the block between the curly braces will be executed
• unless – which will execute the block when a condition expression is evaluated as false – it is the reverse of the if statement, i.e. like an if test with the opposite condition
• switch – well, Perl 5 until 5.10 version has no switch statement to implement a case mechanism in order to avoid the if-elsif-else pattern, but it is easy to build one. Perl 5.9 and 6 come with a built-in switch but it will be spelled "given"

1. evaluate an expression condition in a boolean context
2. if the expression is evaluated as true/false, Perl will execute the next block of statements
3. resume the code with the step 1

• while – executes the block repeatedly as long as a conditional expression is true; the expression is tested before the first iteration of the loop
• do-while – you can use this loop statement when you need to execute a piece of code at least once and then iterate consecutively while the condition is true
• until – is like while, except that it executes the block only if the expression is false; the expression will be tested before the first iteration, too
• do-until – operates similar to do-while, except that it will iterate only if the conditional expression is evaluated false
• for – executes a given set of statements for a certain number of times; in the case of the for statement, the conditional expression consists of three parts (initialization, condition, increment), delimitated by a semicolon symbol
• foreach – operates over a list of values or array by setting consecutively a control variable to each element of the list; some more "perlish" programmers use the word "for" instead of "foreach" because Perl interpreter will easily distinguish between an original for statement (which contains semicolons) and a "for(each)" statement

to see a lot of fully commented examples that help you use the Perl statements and the Perl buit-in functions in your scripts.

• next – is used inside a looping block to skip the following statements of the block when a condition is met, it terminates the current iteration of the loop and then continues with the next iteration, until the end of the loop is reached – it is similar with the continue statement from C language
• last – is like the next control except that it will immediately end the execution of the loop – it is similar with the "break" statement from C language
• redo – returns to the top of the current iteration of the block, without testing any condition

statement if EXPR
statement unless EXPR
statement while EXPR
statement until EXPR
statement foreach EXPR

Please have a look at the following snippet code in order to highlight the topics:

$isPerl = 1;
$a = "Perl Statements\n" if $isPerl == 1;
# it assigns to $a the string value "Perl Statements"
# if the value of $isPerl variable is equal with 1
print $a;
# it prints "Perl Statements"

$c = 2;
$b = 1 unless $c != 2;
# it assigns 1 to $b if $c equals 2
print "b = $b\n"; 
# it prints b = 1

$count = 1;
$count++ while $count < 6;
# it increments $count 5 times
print "Count = $count\n"; #it prints Count = 6 

$count-- until $count == 1;
# it decrements $count 5 times
print "Count = $count\n"; #it prints Count = 1

@numbers = (1, 2, 3, 4, 5);
$_+=5 foreach @numbers;
# increments each element of the array with 5
print "@numbers\n";
# it prints 6 7 8 9 10

to see a lot of fully commented examples that help you use the Perl statements and the Perl buit-in functions in your scripts.