$wrapper = sub {
    $self->unsubscribe($name => $wrapper);
    $cb->(@_);
  };

##</code><code>##

#! perl -slw
use strict;
use Scalar::Util qw[ weaken ];

sub once {
    my $wrapper;
    $wrapper = sub {
        return $wrapper->();
    };
#    weaken $wrapper;
    return $wrapper;
}

once for 1 .. 1e9;