Programming Perl, 4th Edition

Persistent Lexically Scoped Variables: state

A state variable is a lexically scoped variable, just like my. The only difference is that state variables will never be reinitialized, unlike my variables that are reinitialized each time their enclosing block is entered. This is usually so that a function can have a private variable that retains its old value between calls to that function.

state variables are enabled only when the use feature "state" pragma is in effect. This will be automatically included if you ask to use a version of Perl that’s v5.10 or later:

use v5.14;
sub next_count {
    state $counter = 0;  # first time through, only
    return ++$counter;
}

Unlike my variables, state variables are currently restricted to scalars; they cannot be arrays or hashes. This may sound like a bigger restriction than it actually is, because you can always store a reference to an array or hash in a state variable:

use v5.14;
state $bag    = { };
state $vector = [ ];

...
unless ($bag–>{$item}) { $bag–>{$item} = 1 }
...
push @$vector, $item;