How to access subroutine arguments using @_ in Perl?

In Perl, the @_ array is the default mechanism for accessing arguments passed to a subroutine. When you call a subroutine and pass arguments, those values are implicitly stored in the special array @_. Inside the subroutine, you can use @_ to retrieve these arguments by position.

Understanding @_ and Perl Subroutine Arguments

In Perl, all subroutines receive their arguments as a flat list inside the array @_. You can think of this as the subroutine’s "parameter list," but it is actually an array rather than named variables unless you explicitly assign values.

• @_ contains all the arguments passed to the subroutine, indexed starting at 0.
• Each element in @_ is an alias to the actual scalar passed, so modifying $_[0] changes the original argument.
• To handle parameters cleanly, it is common to assign @_ elements to named lexical variables.

Example of Accessing Arguments Using @_

use strict;
use warnings;

# Define a simple subroutine that accepts two arguments
sub greet {
    # Access parameters directly from @_
    my ($name, $age) = @_;  # Assign @_ elements to named variables

    print "Hello, $name! You are $age years old.\n";
}

# Call the subroutine with arguments
greet("Alice", 30);

# You can also access the arguments by direct indexing
sub greet_direct {
    print "Hi, $_[0]! Age: $_[1]\n";
}

greet_direct("Bob", 25);

Output:

Hello, Alice! You are 30 years old.
Hi, Bob! Age: 25

Key Perl Concepts to Note

• Sigils: The @_ array uses the @ sigil indicating it is an array. However, when you want to access a single argument, you use scalar indexing like $_[0], which is a scalar (notice the $ in front of _, e.g., $_[0]).
• Context: @_ is always an array. Assigning it to a list (e.g., my ($a, $b) = @_;) gives you the first two parameters.
• Alias semantics: Each element of @_ is an alias to the argument passed. Be careful if you modify $_[0] directly; it will affect the caller’s variable (if it was a named scalar).
• TMTOWTDI (There's More Than One Way To Do It): You can access parameters directly with indices, assign all at once to a list of lexicals, or even use shift inside the subroutine. For instance:

sub greet_shift {
    my $name = shift;
    my $age  = shift;
    print "Greetings, $name, aged $age\n";
}