Quoth the CPAN:

share() allows you to share($hashref->{key}) and share($arrayref->[idx]) without giving any error message. But the $hashref->{key} or $arrayref->[idx] is not shared, causing the error "lock can only be used on shared values" to occur when you attempt to lock($hashref->{key}) or lock($arrayref->[idx]) in another thread.

I can't say how effective this is, but the following tweak gets a local direct reference to the shared hash instead of following a dereference in the lock call, and doesn't complain.

use strict;
use threads;
use threads::shared;
use warnings;
sub lockit($) {
    my $uber_hashRef = shift ;
    print "Yey\n" if is_shared( $$uber_hashRef{'%y'} ) ;
    my $subHash = $uber_hashRef->{'%y'};
    lock ($subHash) ;
    print "Done!\n";
}
sub genSharedLower($) {
    my $hashRef = shift ;
    $$hashRef{'%y'} = &share({}) ;
}
my %all_shary_for_subs : shared ;
genSharedLower(\%all_shary_for_subs);
print "Yey\n" if is_shared( $all_shary_for_subs{'%y'});
lockit( \%all_shary_for_subs) ;
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Gives:

C:\Documents and Settings\nd250020\Desktop\dev>perl test.pl
Yey
Yey
Done!
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