What is the difference between 'my' and 'our' in Perl?

How does our differ from my and what does our do?

In Summary:

Available since Perl 5, my is a way to declare non-package variables, that are:

• private
• new
• non-global
• separate from any package, so that the variable cannot be accessed in the form of $package_name::variable.

On the other hand, our variables are package variables, and thus automatically:

• global variables
• definitely not private
• not necessarily new
• can be accessed outside the package (or lexical scope) with the qualified namespace, as $package_name::variable.

Declaring a variable with our allows you to predeclare variables in order to use them under use strict without getting typo warnings or compile-time errors. Since Perl 5.6, it has replaced the obsolete use vars, which was only file-scoped, and not lexically scoped as is our.

For example, the formal, qualified name for variable $x inside package main is $main::x. Declaring our $x allows you to use the bare $x variable without penalty (i.e., without a resulting error), in the scope of the declaration, when the script uses use strict or use strict "vars". The scope might be one, or two, or more packages, or one small block.

The PerlMonks and PerlDoc links from cartman and Olafur are a great reference - below is my crack at a summary:

my variables are lexically scoped within a single block defined by {} or within the same file if not in {}s. They are not accessible from packages/subroutines defined outside of the same lexical scope / block.

our variables are scoped within a package/file and accessible from any code that use or require that package/file - name conflicts are resolved between packages by prepending the appropriate namespace.

Just to round it out, local variables are "dynamically" scoped, differing from my variables in that they are also accessible from subroutines called within the same block.

An example:

use strict;

for (1 .. 2){
    # Both variables are lexically scoped to the block.
    our ($o);  # Belongs to 'main' package.
    my  ($m);  # Does not belong to a package.

    # The variables differ with respect to newness.
    $o ++;
    $m ++;
    print __PACKAGE__, " >> o=$o m=$m\n";  # $m is always 1.

    # The package has changed, but we still have direct,
    # unqualified access to both variables, because the
    # lexical scope has not changed.
    package Fubb;
    print __PACKAGE__, " >> o=$o m=$m\n";
}

# The our() and my() variables differ with respect to privacy.
# We can still access the variable declared with our(), provided
# that we fully qualify its name, but the variable declared
# with my() is unavailable.
print __PACKAGE__, " >> main::o=$main::o\n";  # 2
print __PACKAGE__, " >> main::m=$main::m\n";  # Undefined.

# Attempts to access the variables directly won't compile.
# print __PACKAGE__, " >> o=$o\n";
# print __PACKAGE__, " >> m=$m\n";

# Variables declared with use vars() are like those declared
# with our(): belong to a package; not private; and not new.
# However, their scoping is package-based rather than lexical.
for (1 .. 9){
    use vars qw($uv);
    $uv ++;
}

# Even though we are outside the lexical scope where the
# use vars() variable was declared, we have direct access
# because the package has not changed.
print __PACKAGE__, " >> uv=$uv\n";

# And we can access it from another package.
package Bubb;
print __PACKAGE__, " >> main::uv=$main::uv\n";

Coping with Scoping is a good overview of Perl scoping rules. It's old enough that our is not discussed in the body of the text. It is addressed in the Notes section at the end.

The article talks about package variables and dynamic scope and how that differs from lexical variables and lexical scope.

The perldoc has a good definition of our.

Unlike my, which both allocates storage for a variable and associates a simple name with that storage for use within the current scope, our associates a simple name with a package variable in the current package, for use within the current scope. In other words, our has the same scoping rules as my, but does not necessarily create a variable.

my is used for local variables, whereas our is used for global variables.

More reading over at Variable Scoping in Perl: the basics.

I ever met some pitfalls about lexical declarations in Perl that messed me up, which are also related to this question, so I just add my summary here:

1. Definition or declaration?

local $var = 42;
print "var: $var\n";

The output is var: 42. However we couldn't tell if local $var = 42; is a definition or declaration. But how about this:

use strict;
use warnings;

local $var = 42;
print "var: $var\n";

The second program will throw an error:

Global symbol "$var" requires explicit package name.

$var is not defined, which means local $var; is just a declaration! Before using local to declare a variable, make sure that it is defined as a global variable previously.

But why this won't fail?

use strict;
use warnings;

local $a = 42;
print "var: $a\n";

The output is: var: 42.

That's because $a, as well as $b, is a global variable pre-defined in Perl. Remember the sort function?

2. Lexical or global?

I was a C programmer before starting using Perl, so the concept of lexical and global variables seems straightforward to me: it just corresponds to auto and external variables in C. But there're small differences:

In C, an external variable is a variable defined outside any function block. On the other hand, an automatic variable is a variable defined inside a function block. Like this:

int global;

int main(void) {
    int local;
}

While in Perl, things are subtle:

sub main {
    $var = 42;
}

&main;

print "var: $var\n";

The output is var: 42. $var is a global variable even if it's defined in a function block! Actually in Perl, any variable is declared as global by default.

The lesson is to always add use strict; use warnings; at the beginning of a Perl program, which will force the programmer to declare the lexical variable explicitly, so that we don't get messed up by some mistakes taken for granted.