What is the difference between my and local in Perl?

Question

I am seeing both of them used in this script I am trying to debug and the literature is just not clear. Can someone demystify this for me?

Answer 1

Dynamic Scoping. It is a neat concept. Many people don't use it, or understand it.

Basically think of my as creating and anchoring a variable to one block of {}, A.K.A. scope.

my $foo if (true); # $foo lives and dies within the if statement.

So a my variable is what you are used to. whereas with dynamic scoping $var can be declared anywhere and used anywhere. So with local you basically suspend the use of that global variable, and use a "local value" to work with it. So local creates a temporary scope for a temporary variable.

$var = 4;
print $var, "\n";
&hello;
print $var, "\n";

# subroutines
sub hello {
     local $var = 10;
     print $var, "\n";
     &gogo; # calling subroutine gogo
     print $var, "\n";
}
sub gogo {
     $var ++;
}

This should print:

4
10
11
4

Answer 2

I think the easiest way to remember it is this way. MY creates a new variable. LOCAL temporarily changes the value of an existing variable.

Answer 3

Quoting from Learning Perl:

But local is misnamed, or at least misleadingly named. Our friend Chip Salzenberg says that if he ever gets a chance to go back in a time machine to 1986 and give Larry one piece of advice, he'd tell Larry to call local by the name "save" instead.[14] That's because local actually will save the given global variable's value away, so it will later automatically be restored to the global variable. (That's right: these so-called "local" variables are actually globals!) This save-and-restore mechanism is the same one we've already seen twice now, in the control variable of a foreach loop, and in the @_ array of subroutine parameters.

So, local saves a global variable's current value and then set it to some form of empty value. You'll often see it used to slurp an entire file, rather than leading just a line:

my $file_content;
{
    local $/;
    open IN, "foo.txt";
    $file_content = <IN>;
} 

Calling local $/ sets the input record separator (the value that Perl stops reading a "line" at) to an empty value, causing the spaceship operator to read the entire file, so it never hits the input record separator.

Answer 4

Well Google really works for you on this one: http://www.perlmonks.org/?node_id=94007

From the link:

Quick summary: 'my' creates a new variable, 'local' temporarily amends the value of a variable.

ie, 'local' temporarily changes the value of the variable, but only within the scope it exists in.

Generally use my, it's faster and doesn't do anything kind of weird.

Answer 5

&s;

sub s()
{
    local $s="5";
    &b;
    print $s;
}

sub b()
{
    $s++;
}

The above script prints 6.

But if we change local to my it will print 5.

This is the difference. Simple.

Answer 6

Look at the following code and its output to understand the difference.

our $name = "Abhishek";

sub sub1
{
    print "\nName = $name\n";
    local $name = "Abhijeet";

    &sub2;
    &sub3;
}

sub sub2
{
    print "\nName = $name\n";
}

sub sub3
{
    my $name = "Abhinav";
    print "\nName = $name\n";
}


&sub1;

Output is :

Name = Abhishek

Name = Abhijeet

Name = Abhinav