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# Recursion in Perl

• Last Updated : 27 Jun, 2021

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 \$_, \$_ 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` `= ``\$_``;` `# 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

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