The short version for the impatient

• my() creates a local variable
• our() creates a package variable
• local() temporarily changes the local value of a global variable
• The above is mostly true.

The long version for the irrepressibly quixotic

my()

my declares the listed variable to be local to the enclosing block, file, or eval. That is to say its scope is local. This kind of variable is known as a lexical variable. Note that lexical variables are hidden from subroutines which are called from within the enclosing block. This is known as lexical scoping.

our()

our creates an alias to a package variable. The alias is local to the enclosing block, file, or eval. That is to say the alias is lexically scoped just like any lexical variable. However a package variable belongs to a package. It can be accessed from anywhere if you use its fully qualified name. Here are two examples of fully qualified package variables:

$main::a
%MyPackage::boop

Note that package variables are also global variables.

local()

local gives temporary values to global variables. It does not create a local variable. It is most commonly used when you want to locally modify a global variable such as one of the punctuation variables. For example:

{ 
    local $| = 1; # enable autoflush for STDOUT
    say "hi mom";
}

local modifies the listed variable to be local to the enclosing block, file, or eval – AND to any subroutine called from within that block. This is known as dynamic scoping.

Sources

For a more complete understanding I recommend perldoc perlfunc and especially perldoc perlsub. Also the following links may be helpful:

• https://metacpan.org/pod/perlfunc
• https://metacpan.org/pod/perlsub
• http://perlmaven.com/package-variables-and-lexical-variables-in-perl
• http://www.perlmonks.org/?node_id=95813