Recursion in Perl

Recursion is a mechanism when a function calls itself again and again till the required condition is met. When the function call statement is written inside the same function then such a function is referred to as a recursive function.
The argument passed to a function is retrieved from the default array @_ whereas each value can be accessed by $_[0], $_[1] and so on.
Example 1: The example below finds factorial of a number. 

Factorial of any number n is (n)*(n-1)*(n-2)*....*1.
e.g.:
4! = 4*3*2*1 = 24
3! = 3*2*1 = 6
2! = 2*1 = 2
1! = 1
0! = 0

Perl

#!/usr/bin/perl

# Perl Program to calculate Factorial
sub fact
{

# Retrieving the first argument
# passed with function calling
my $x = $_[0];

# checking if that value is 0 or 1
if ($x == 0 || $x == 1)
{
    return 1;
}

# Recursively calling function with the next value
# which is one less than current one
else
{
    return $x * fact($x - 1);
}
}

# Driver Code
$a = 5;

# Function call and printing result after return
print "Factorial of a number $a is ", fact($a);

Here is how the program works :
Step 1- When the value of scalar a is 0 or 1, the function will return 1 because the value of both 0! and 1! is 1. 
Step 2- When the value of scalar a is 2 then fac(x-1) makes a call to fac(1) and this function returns 1. 
So, it is 2*factorial(1) = 2*1 = 2.So, it will return 2. 
Step 3- Similarly when higher values are passed to function at every call argument value decreases by 1 and computes till the value reaches 1.
Example 2: Example below computes the Fibonacci series till a given number. 

Perl

#!/usr/bin/perl

# Perl Program to print Fibonacci series
sub fib
{
    # Retrieving values from the parameter
    my $x = shift;
    my $y = shift;

    # Number till which the series is to be printed
    my $n = shift;

    # Check for the end value
    if ($y > $n)
    {
        return 1;
    }

    # Printing the number
    print " $y";

    # Recursive Function Call
    fib($y, $x + $y, $n);
}

# Driver Code

# Number till which series is to be printed
$a = 5;

# First two elements of the series
$c = 0;
$d = 1;

print "$c";

# Function call with required parameters
fib($c, $d, $a);

Here is how the program works :
Step 1- Function fib() is called with 3 parameters starting values which will be 0 and 1 while $n is a number till which a series is to be printed 
Step 2- These values are transferred in the form of an array whose values are retrieved with the use of shift. 
Step 3- At each call first two values are retrieved using shift and these values are stored in scalars x and y. Now these two values are added to get the next value in the series. This step continues till the value reaches the ending value provided by the user