In this short free tutorial, I’ll show you how you can use the Perl if statement in your Perl scripts. The if statement let you execute a block of statements between two curly braces if a boolean condition is evaluated true, otherwise the execution continues with the following block, either an elsif/else block or after the end of the if statement.

You may scope some of your variables with my keyword in the control condition and in this case the scope of the variables extends from its declaration through the rest of the Perl if statement, including any elsif or else clause.

We have two syntax forms for this conditional statement.

STATEMENT if (EXPR)

In this first form we used if as a modifier of a statement. EXPR represents a condition which must be evaluate in order to execute the Perl if statement.

By writing code like this, you can put the important part of the statement at the beginning, in order to monitor it. The next code sample shows you how to use it when you need to print the values of certain variables in order to debug your Perl script:

$debug = 1;
$a = 1; $b = 2;
# … some statements here
print "a = $a\n" if $debug;   # check the $a value
# … other statements
print "b = $b\n" if $debug;   # check the $b value
# the rest of script statements

The next example will show you how to use the regular expressions in the Perl if condition (the =~ operator):

$v = "An example with the if modifier";
print "\\\\$v contains the word \'example\'\n" if ($v =~ /example/);
# it prints: $v contains the word 'example'
$v = "";

The second syntax form is more general:

if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK

• EXPR represents a Boolean conditional expression
• the elsif and else clauses are optional and are used if you want to check a certain sequence of conditional expressions and find out which one of them is true
• BLOCK consists of one or more statements enclosed in curly braces; in the construction of a block, the curly braces are always required.

if (EXPR) BLOCK

$a = -1;
if($a < 0) {
  $a += 2;  
}
print "a = $a\n";  # it outputs a = 1

It is possible that EXPR doesn’t represent a condition, like in the following example:

if(1) {
  $a = 1;
}

if (EXPR) BLOCK else BLOCK

Let’s look at a simple example where we need to find the maximum of two numbers:

$a = 15;
$b = 27;
if($a > $b) {
  print "max($a, $b) = $a\n";
} else {
  print "max($a, $b) = $b\n";
}
# it prints: max(15, 27) = 27

Well, there is a shorter way to do this by using the (?) logical ternary operator - also called the conditional operator. I remind you briefly that the ternary operator (named ternary because it uses three operands) is like an if-else compound statement both included into an expression. Please note that this operator is an expression that returns a value based on the conditional sentence it evaluates. It can be used by following the syntax:

EXPR ?  true-value : false-value

With the ternary operator, we can rewrite the above code like so:

$a = 15; $b = 27;
print "max($a, $b) = ", $a > $b ? $a : $b,"\n";

Now I’ll discuss a bit about the case when in a Perl if statement we have an empty block after the expression, like in the following snippet code example:

$a = 12; $b = -25;
if($a*$b >= 0 && $a + $b  >= 0) {
  # do nothing
} else {
  print "at least $a or $b is negative\n";
}

$a = 12; $b = -25;
unless($a*$b >= 0 && $a + $b  >= 0) {
  print "at least $a or $b is negative\\n";
}

Another way is to rewrite the conditional expression enclosed between the parentheses that follow the if keyword: ($a*$b >= 0 && $a + $b >= 0). This is a boolean expression and as you expect Perl provides you with the logical AND operator (&&) and the logical OR operator (||) in order to manipulate the boolean expressions. When you use the && and || operators be careful not to include any spaces between the symbols ("& &" or "| |" is wrong) because you’ll get something you couldn’t expect.

So than we can rewrite the above condition in order to get rid of the empty block:

$a = 12; $b = -25;
if($a*$b < 0) {
  print "at least $a or $b is negative\n";
}

if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK

The next snippet code shows you an example about how to use if in connection with the elsif and else clauses:

chomp ($line = <STDIN>);
$line = lc($line);
if($line eq "index"){
  print "index function\n";
} elsif ($line eq "chomp") {
  print "chomp function\n";
} elsif ($line eq "hex") {
  print "hex function\n";
} elsif ($line eq "substr") {
  print "substr function\n";
} else {
  print "$line: unknown function\n";
}

Please note that the Perl interpreter must evaluate all the elsif alternatives you provide in an if statement. If there are too many, you need to reconsider a more efficient way to write the code, like emulate a case or switch statement (in Perl 5.10 you can use the given function).