Marcel Grünauer

Suppose that a subroutine gets called several times and that in the subroutine you need to use some computational result that stays the same for every call. You could use a state variable or an our variable.

sub foo {
    # ...

    state $some_value //= ... some computation ...;

    # ...
}

Using the defined-or operator, the value will only be computed the first time the subroutine is called.

But what if the computation takes several statements? You could move it to its own subroutine and call it with a single statement, or you might resort to something like:

# NOT IDIOMATIC
sub foo {
    # ...

    state $some_value;
    unless (defined $some_value) {
        # ... some more ...
        # ... intensive ...
        # ... computation ...
        $some_value = ...;
    }

    # ...
}

The problem is that we need to write $some_value three times.

But a more idiomatic way is to use an anonymous subroutine and call it in-place:

sub foo {
    # ...

    state $some_value //= sub {
        # ... some more ...
        # ... intensive ...
        # ... computation ...
    }->();

    # ...
}