I can confirm this behaviour on perl 5.18.1 with Scalar::Util 1.35. Looks like a bug to me.

Note that the correct behaviour can be obtained using the experimental “lexical subroutines” feature, introduced in perl 5.18.0 (see perlsub):

#! perl
use strict;
use warnings;
use feature      'lexical_subs';
use Scalar::Util 'weaken';

my $ref;

{
    my sub frobnicate
    {
        print 42;
    }

    my $local_ref = \&frobnicate;
#   $ref = $local_ref;
    weaken($ref = $local_ref);
}

$ref->();
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Output:

12:49 >perl 775_SoPW.pl
The lexical_subs feature is experimental at 775_SoPW.pl line 10.
Can't use an undefined value as a subroutine reference at 775_SoPW.pl 
+line 20.

12:49 >
[download]

Hope that helps,