Actually it looks like if the hash itself is not shared, any changes to keys will not affect other running threads. This includes deleting keys (the key/value will still be there in other threads). Only changes to the shared values will affect other running threads (including undefining them).

use strict;
use threads;
use threads::shared;

my %hash = (
    'a' => 'foo',
    'b' => 'foo',
    'c' => 'foo',
    'd' => 'foo'
);
share($hash{$_}) foreach('a'..'f');
my $tid = 0;
foreach my $key('a'..'f') {
    if(exists $hash{$key}) {
        if(defined $hash{$key}) {
            print "$key => $hash{$key} in thread $tid\n";
        }
        else {
            print "$key => undef in thread $tid\n";
        }
    }
    else {
        print "$key does not exist in thread $tid\n";
    }
}

my $th1 = async {
    $tid = 1;
    print "thread $tid started\n";
    sleep 2;
    foreach my $key('a'..'f') {
        if(exists $hash{$key}) {
            if(defined $hash{$key}) {
                print "$key => $hash{$key} in thread $tid\n";
            }
            else {
                print "$key => undef in thread $tid\n";
            }
        }
        else {
            print "$key does not exist in thread $tid\n";
        }
    }
    print "delete b from thread $tid\n";
    delete $hash{'b'};
    print "undefine d from thread $tid\n";
    undef $hash{'d'};
    print "setting f => foo in thread $tid\n";
    $hash{'f'} = 'foo';
};
my $th2 = async {
    $tid = 2;
    print "thread $tid started\n";
    sleep 3;
    foreach my $key('a'..'f') {
        if(exists $hash{$key}) {
            if(defined $hash{$key}) {
                print "$key => $hash{$key} in thread $tid\n";
            }
            else {
                print "$key => undef in thread $tid\n";
            }
        }
        else {
            print "$key does not exist in thread $tid\n";
        }
    }
};
sleep 1;
print "delete a from thread $tid\n";
delete $hash{'a'};
print "undefine c from thread $tid\n";
undef $hash{'c'};
print "setting e => foo in thread $tid\n";
$hash{'e'} = 'foo';
$th1->join();
$th2->join();
foreach my $key('a'..'f') {
    if(exists $hash{$key}) {
        if(defined $hash{$key}) {
            print "$key => $hash{$key} in thread $tid\n";
        }
        else {
            print "$key => undef in thread $tid\n";
        }
    }
    else {
        print "$key does not exist in thread $tid\n";
    }
}
[download]

a => foo in thread 0
b => foo in thread 0
c => foo in thread 0
d => foo in thread 0
e => undef in thread 0
f => undef in thread 0
thread 1 started
thread 2 started
delete a from thread 0
undefine c from thread 0
setting e => foo in thread 0
a => foo in thread 1
b => foo in thread 1
c => undef in thread 1
d => foo in thread 1
e => foo in thread 1
f => undef in thread 1
delete b from thread 1
undefine d from thread 1
setting f => foo in thread 1
a => foo in thread 2
b => foo in thread 2
c => undef in thread 2
d => undef in thread 2
e => foo in thread 2
f => foo in thread 2
a does not exist in thread 0
b => foo in thread 0
c => undef in thread 0
d => undef in thread 0
e => foo in thread 0
f => foo in thread 0
[download]