You are using two techniques. I don't know which you are asking about.

• You are using closures to privatise attributes. (Contrary to what others have said, you don't have "a closure". Each of your methods are closures.) I don't know if using closures for this purpose has a name.

• You have per-object methods instead of classes. JavaScript uses this. It's called Prototype-based programming. Except you don't actually support prototypes.

By the way, why create %internal at all?

sub new_counter {
    my ($start) = @_;

    my $count = $start;

    return {
        add => sub {
            my ($value) = @_;
            $count += $value;
        },

        get => sub {
            return $count;
        },
    };
}
[download]

Or even

sub new_counter {
    my ($count) = @_;

    ...
}
[download]