Describe the concept of scope and variable visibility in Perl.

In Perl, scope refers to the visibility and lifetime of variables within a program. It determines where and for how long a variable can be accessed. Understanding scope and variable visibility is crucial for writing reliable and maintainable Perl code. Let's dive into the concept of scope in Perl:

1. Global Scope:

* Variables declared outside of any subroutine or block have global scope.
* Global variables are accessible from anywhere in the program.
* They retain their values throughout the entire execution of the program.
* Example:
```
perl`my $global_var = 10;

sub foo {
print $global_var; # Can access global variable
}`
```
2. Lexical (or Private) Scope:

* Variables declared within a block, such as a subroutine or loop, have lexical scope.
* Lexical variables are accessible only within the block where they are declared.
* They are created when the block is entered and destroyed when the block is exited.
* Example:
```
perl`sub foo {
my $lexical_var = 20;
print $lexical_var; # Can access lexical variable within the block
}

foo();
print $lexical_var; # Error: Cannot access lexical variable outside the block`
```
3. Scope Modifiers:

* In Perl, you can modify the default scope of variables using `my`, `our`, or `local` keywords.
* `my` declares a lexical variable with a limited scope.
* `our` declares a global variable that is accessible within the current package.
* `local` creates a temporary localized value for a global variable within a specific block, but the original value is restored after the block.
* Example:
```
perl`our $global_var = 10;

sub foo {
my $lexical_var = 20;
local $global_var = 30; # Temporarily modifies the value of the global variable
print $lexical_var; # Can access lexical variable within the block
print $global_var; # Can access modified global variable within the block
}

foo();
print $lexical_var; # Error: Cannot access lexical variable outside the block
print $global_var; # Original global variable value is restored`
```
4. Nested Scopes:

* Perl allows nesting of scopes, where inner scopes can access variables from outer scopes.
* Inner scopes can shadow variables with the same name in outer scopes, effectively creating a new variable.
* Example:
```
perl`my $x = 10;

sub outer {
my $x = 20; # Shadows the outer $x variable
print $x; # Prints the value of the inner $x
{
my $x = 30; # Creates a new variable within the inner block
print $x; # Prints the value of the innermost $x
}
print $x; # Prints the value of the inner $x
}

outer();
print $x; # Prints the value of the outer $x variable`
```

Understanding scope and variable visibility in Perl is essential to avoid naming conflicts, manage variable lifetime, and ensure the correct access and manipulation of data within your program. By appropriately defining and managing the scope of variables, you can write more reliable and maintainable Perl code.