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.

Another approach using Recursion to print Fibonacci Series in Perl – 

Perl

sub fibonacci {
   my($num) = @_;
   if ($num < 2) { # base case
      return $num;
   }
   return fibonacci($num-1) + fibonacci($num-2);
}

print("Enter the number till the series will be printed : ");
$num = <>;

for ($i = 0; $i < $num; $i++){
    print(fibonacci($i)," ");
}

Output –