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Flow Control

In Perl, like in C++, all simple statements must end with a semicolon. A compound statement is a sequence of statements and is commonly referred to as a block.  A block has its own scope, and is usually surrounded by curly {} braces. Perl uses control structures such as if ... else and while in a way that is very similar to other languages like C, C++ and Java. However, it gives you more flexibility with their syntax that these other languages.

Conditionals

If statements

An if statement in Perl looks almost identical to the if statements you are already familiar with. It has the general form:

if (condition) block { elsif (condition) block } [ else block ]

where { x } means that x can occur zero or more times, and [ x ] means that x can occur zero or one time. Note that, contrarily to C, C++ and Java, the block must be enclosed in curly braces, even if it is a single statement. In other words,

if ($a > 5) print "TRUE\n";

is illegal in Perl, and instead we need to use

if ($a > 5) { print "TRUE\n"; }

Unless statements

There is also unless statement that works in exactly the same way as the if statement, except that the first condition is negated. That is, the statement:

if ($a > 5) 
{ 
    print "TRUE\n"; 
} 
elsif ($a == 3) 
{ 
    print "MAYBE\n";
} 
else 
{
    print "FALSE\n";
}

is the same as the statement:

unless ($a <= 5) 
{ 
    print "TRUE\n"; 
} 
elsif ($a == 3) 
{ 
    print "MAYBE\n";
} 
else 
{
    print "FALSE\n";
}

Note that there is no elsunless keyword.

Switch statements

Unlike the if and unless statements, there is no official switch statement. Fortunately, due to the flexibility of Perl, there are many ways to create a switch statement. The easiest, but perhaps not the most attractive, way to mimic the behaviour of a switch statement is to use a series of if-elsif statements nested inside of a for loop. Another alternative, which resembles C/C++ switch statements more closely, is to use a label, a block, some if statements, and the last keyword. The label, which is SWITCH in the example below, is simply used to identify the block. This facilitates the use of control statements such as last, which we use here, next, and redo. The last keyword is like the C/C++ break keyword. It is used to immediately terminate execution in the labeled block, or if no label is given, the innermost block. Here is a sample program:

@factor = ("quadruple", "halve", "double", "triple");

for ($i = 0; $i < @factor.""; $i++)
{
    $amount = 1000;

    SWITCH: {
	if ($factor[$i] eq "double") 
	{ 
	    $amount *= 2; 
	    last SWITCH; 
	}
	if ($factor[$i] eq "triple") 
	{ 
	    $amount *= 3; 
	    last SWITCH; 
	}
	if ($factor[$i] eq "quadruple") 
	{ 
	    $amount *= 4; 
	    last SWITCH; 
	}
	
	print "I do not know how to $factor[$i].\n";
	$amount = 0;
    }

    print "You wanted to $factor[$i] the amount.\n";
    print "The new amount is $amount.\n\n";
}

You wanted to quadruple the amount.
The new amount is 4000.
 
I do not know how to halve.
You wanted to halve the amount.
The new amount is 0.
 
You wanted to double the amount.
The new amount is 2000.
 
You wanted to triple the amount.
The new amount is 3000.

For more examples of how to create a switch statement, please consult the following pages: http://www.perl.com/CPAN-local/doc/manual/html/pod/perlsyn.html#Basic_BLOCKs_and_Switch_Statemen, http://language.perl.com/misc/fmswitch, or http://www.perl.com/CPAN-local/doc/manual/html/pod/perlfaq7.html#How_do_I_create_a_switch_or_case.

Loops

Perl has a while loop and a for loop that are almost identical to their C++ or Java counterparts, the only difference being that the body of the loop must be enclosed between curly braces (just like the blocks in the if statement). The other two looping constructs that it supports are the do...until loop, and the foreach loop.

do...until loops

The do...until loop is similar to the do...while loop in C++, except that the condition is negated. Its general form is: do block until (condition)

which is equivalent to do block while (!condition)

in C++.

foreach loops

A foreach loop has the form: foreach $scalar (@array) block

and executes block as many times as there are elements in @array, with $scalar successively equal to the first, second, third, etc, element of the array. It is suggested that you use the foreach construct in favour of the regular for and while constructs whenever it is possible. In most cases, this makes for a smaller program and eliminates the notorious "off-by-one-error".

Note that the keywords for and foreach are, in fact, interchangeable. However, for readability reasons we recommend that you use them as described in this section.

Statement modifiers

Finally, Perl does give statements one unexpected option: they can have one of the following modifiers, placed after the statement, and not to be confused with the control structures described above (even though the two are closely related).

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