The author must have done this for some reason...

Because in the real code, unlike your simplification of it, the weakened sub reference forms a closure over itself:

  $wrapper = sub {
    $self->unsubscribe($name => $wrapper);
    $cb->(@_);
  };
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Ie. It contains a circular reference which would leak memory without the weakening.

For example, with the weaken commented, this leaks like a sieve:

#! 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;
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