use threads;
use threads::shared;
my $var           = 1;
my $svar : shared = 2;
my %hash : shared;

 ... create some threads ...

$hash{a} = 1;       # all threads see exists($hash{a}) and $hash{a} == 1
$hash{a} = $var     # okay - copy-by-value: same effect as previous
$hash{a} = $svar    # okay - copy-by-value: same effect as previous
$hash{a} = \$svar   # okay - a reference to a shared variable
$hash{a} = \$var    # This will die
delete $hash{a}     # okay - all threads will see !exists($hash{a})