Local and Global variables in perl

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In terms of scoping, there are three kinds of variables in Perl.

  • Lexical variables are lexically scoped, which means they are only visible in the current lexical scope (basically file or block).

  • Package variables, on the other hand, can be used using their qualified form (e.g. $Foo::x) from anywhere in the interpreter, and they can be used without qualification by any code that shares the variable’s package.

  • Certain package variables are visible without qualification anywhere in the interpreter. These include punctuation vars and a few named vars such as @ARGV and STDOUT. For example, $x refers to $Foo::x when in package Foo and $Bar::x when in package Bar (assuming no lexical var named $x is in scope), but $_ always refers to $::_.

Variables are destroyed when they are no longer referenced.

  • Lexical variables are usually destroyed when the lexical scope is exited.

  • Package variables are usually destroyed when the program exits.

Here are ways to create variable.

  • my and state create a lexical variable.

  • our creates a lexical variable that is aliased to the variable of the same name in the current package. In other words, our $x; is equivalent to my \$x = \$Foo::x; when in package Foo.

  • Package variables are created on use.

local doesn’t create any variables. It simply backs up a variable until the current lexical scope is destroyed. It is restored from its backed-up value at that point.

my does the same thing.

No. local does not change the scope of a variable. While a lexical variable is only visible in a lexical scope, a localized package variable is still visible across the entire interpreter.

$x = 123;
sub foo { print "$x\n"; }
{ local $x = 456; foo(); }  # 456
foo();                      # 123


$x = 123;
sub foo { print "$x\n"; }
{ my $x = 456; foo(); }   # 123
foo();                    # 123

What else for local

local is primarily used to approximate the functionality of my for variables that cannot otherwise be declared lexically.

Historically, that was all variables. Since 5.6, only punctuation variables cannot be declared lexically.

What is “global” variable?

A global variable is a variable that can seen globally.

All package variables can be seen by any code in the interpreter, so they’re all global.

Or are they? To see them from other packages, you need to qualify them. Are $x and $Foo::x the same variable?

To some, global variables refers to the set of package variables you can use unqualified. It means that package changes the set of global variables. And since the package directive is usually used on a file-basis, that means file-level lexicals are also effectively global by this definition. And they are indeed called that sometimes.

But if the package changes the set of variables that are global, then they’re not really global, are they? So think some people, which only consider punctuation variables (e.g. $_) and the few named variables that can be used unqualified from anywhere (*::STDOUT) to be global.

In short, it’s a pretty useless term.

Is it possible to add the my scoped variables in @EXPORT array and use it in another packages?

No. @EXPORT is used by Exporter. Exporter would not be able to find anything but package symbols (since files are compiled in fresh lexical scopes), so @EXPORT must only contain package symbols.

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