The modified version is broken also. As demonstrated by the attached code.

The number of elements in @order in each thread is "weird". For example, 6028, 11159, 22541 and 36060. It should be 17576, 17576, 17576 and 17576.

        my @order = shuffle keys %hash;
[download]

should be

        my @order;
        {
            lock %hash;
            @order = shuffle keys %hash;
        }
[download]

It's the test harness that's not thread-safe.

Indeed, but the final death knell is that using a mirror structure of individual sems is 3 times slower than locking the entire hash!

Lock the hash:

#! perl -slw
use strict;
use Data::Dump qw[ pp ];
use Time::HiRes qw[ time ];
use List::Util qw[ shuffle ];
use threads;
use threads::shared;

our $START //= 'aaa';
our $END   //= 'zzz';

my %hash :shared = map{ $_ => 0 } $START .. $END;

my $start = time;
$_->join for map{
    async{
        my @order;
        {
            lock %hash;
            @order = shuffle keys %hash;
        }
        for ( @order ) {
            lock %hash;
            ++$hash{ $_ };
        }
    }
} 1 .. 4;

printf "Lock hash took %.3f seconds\n", time() - $start;

my @fails = grep $_ != 4, values %hash;
warn @fails . " fails\n" if @fails;

__END__
C:\test>909437-1
Lock hash took 1.093 seconds
[download]

Locking scalars:

#! perl -slw
use strict;
use Data::Dump qw[ pp ];
use Time::HiRes qw[ time ];
use List::Util qw[ shuffle ];
use threads;
use threads::shared;

our $START //= 'aaa';
our $END   //= 'zzz';

my %sems :shared = map{ my $n :shared = 0; $_ => \$n } $START .. $END;
my %hash :shared = map{ $_ => 0 } $START .. $END;

my $start = time;
$_->join for map{
    async{
        my @order;
        {
            lock %hash;
            @order = shuffle keys %hash;
        }
        for ( @order ) {
            lock ${ $sems{ $_ } };
            ++$hash{ $_ };
        }
    }
} 1 .. 4;

printf "Lock scalars ref took %.3f seconds\n", time() - $start;

my @fails = grep $_ != 4, values %hash;
warn @fails . " fails\n" if @fails;

__END__
C:\test>909437-3
Lock scalars ref took 4.667 seconds
[download]

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

You seem to have forgotten to attach the code?

Update: ah, while it doesn't use my code, it does seem to indicate there's a race condition in my code.