Perl’s **“Use of uninitialized value”** warning is a run-time warning encountered when a variable is used before initialization or outside its scope. The warning does *not* prevent code compilation.

To view possible warnings, you will need to include the `warnings` library in your program, as shown below:

```Perl
use warnings;
```

<h2>Warning format</h2>

The format of the “Use of uninitialized value” warning is shown below:

```Perl
Use of uninitialized value %s
```

The placeholder, `%s`, represents the variable that was not initialized with a value.

<h2>Example</h2>

The following example shows how the “Use of uninitialized value” warning arises in Perl:


#!/usr/bin/perl

use warnings;

# declare variable
$name = "Bob";

# print variable
print "Name = $name\n";
print "Age = $age\n";


# intialize value for age
$age = 23;
print "Age = $age\n";

<h2>Explanation</h2>

First, the variable `$name` is initialized and output. Since `name` already has a value, the `print` statement in line $9$ does not cause any problems.

However, the code proceeds to print the variable `$age`, which has not been initialized. Consequently, the `print` statement in line $10$ results in the *“Use of uninitialized value"* warning. The warning alerts the user to the variable's name that caused the issue, i.e. `$age`.

Since `$age` is initialized in line $14$, the final `print` statement does not prompt any warnings.

<h2>Explanation</h2>

First, the variable `＄name` is initialized and output. Since `name` already has a value, the `print` statement in line $9$ does not cause any problems.

However, the code proceeds to print the variable `＄age`, which has not been initialized. Consequently, the `print` statement in line $10$ results in the *“Use of uninitialized value"* warning. The warning alerts the user to the variable's name that caused the issue, i.e. `＄age`.

Since `＄age` is initialized in line $14$, the final `print` statement does not prompt any warnings.

What is the 'Use of uninitialized value' warning in Perl?

Perl’s “Use of uninitialized value” warning appears when a variable is used before initialization.